The remark in question which triggered this particular avenue of introspection is as follows:
While this can just as easily be dismissed as simply detracting from the discussion and rerouting a thread on spell focus changes to "buff fighters pl0xz", I took note of this comment for two reasons. One, I rarely see any PCs that don't have the ability to use magic in some way, shape, or form; Two, I am deeply loath to do so myself. This got me to thinking - why do I feel such a sinking regret in the pit of my gut every time I commit myself to a character concept that doesn't at least have a thorough investment into UMD? The conclusion I came to was, in essence, that playing a character who has access to magic simply lets you do more by a nearly logarithmic measure, in terms of the sheer roleplayability alone.
Before I continue, I will note that by "Mage" I refer to any character that has access to innate casting. Note that this includes wizards, clerics, paladins, rangers, assassins, and any other character which has access to an innate spell list. Though, I will point out that this is ironically the greater majority of classes that players have access to, making it statistically more likely to run into a mage or not by sheer randomization alone.
Completely disregarding mechanical power, which is its own unique can of worms, mages and characters who have access to magical ability simply have more options they can present in any given narrative than an equivalent mundane character. Any storyline available to a mundane character, save for that of "The Mundane Hero and their struggle against/with magic", can similarly be played out by a character who has access to magic, albeit with a likely different result. Moreover, a mage has access to plots that a mundane character simply is not equipped to address: A mundane character is very unlikely to be able to address the portal to the far realm which had opened on his doorstep unless he has access to some manner of magical prowess, such as a scroll of Disjunction or Banishment, or similar magical ability accessed via UMD. At best, without these, a mundane character will be able to chop and/or shoot at the gibbering horrors spilling forth from the dread rift while the mages sit in the back and actually fix the problem.
This gets only more problematic as the scale of the problem increases. Five goblins show up to town? The wizard doesn't need to lift a finger because the fighter has it handled, but could just as easily clap his hands and cast Fireball (or Burning Hands, for that matter) and address it with equal finality. An army of orcs shows up to town, and the fighter might be able to challenge the chieftain to a duel, while the Wizard can, again, clap twice and cast Wail of the Banshee and decimate the greenskin horde. Woe be unto the army who encounters an ESF: Evocation cleric who has the Cave domain, who will not even need a second casting of Earthquake to send the survivors scattering. Now, say that some truly terrible foe appears - an Ancient Red Dragon, for example. Unless it makes the tragic mistake of landing within our poor fighter's reach, he is utterly at the beast's mercy unless he has some way of closing that vertical distance between the two.
Now, this isn't limited to narrative problems of a combative nature. What is your average mundane character to do when presented with a mystery? I hope you really topped off your Search and Spot skills, because if you didn't then you're sitting there twiddling your thumbs while the Wizard starts casting Detect Magic and any number of other information-gathering magics. If the DM hasn't deliberately set up the plot to obfuscate the mage's attempts to seek out information - such as the Cleric literally resurrecting the murder victim and asking who their killer was - it will be solved swiftly and with all the finality that magic has to offer. Similarly, matters of diplomacy are infinitely aided by the correct application of a friendly bard nearby, whose natural charisma paired with their oratory magics boosting their Persuade checks will make even the most bristly of neighbors come to a peaceable solution, while a purely mundane character would need to work and invest easily ten times as much to get a remotely similar result. These statements completely disregard the narrative flavor that an odd cantrip provides in helping with the general business of your character - being able to heat your food with a simple spark cantrip to add narrative flair, or stating that you've changed your clothes with a simple transmutation spell to explain your sudden change of wardrobe instead of handwaving away the hundred tailors kept in the closet of the Wave and Serpent to hasten everyone's clothing changes, for example.
Ultimately, though, I don't think there is anything to be done about this - Nor do I think that there is, frankly, anything which can be done about this short of giving everyone access to UMD, which would invalidate ~40% of the appeal of the Rogue and Bard classes altogether. Ultimately one must stick to their concept as best as they are able, and roleplay it as long as they're willing to maintain its sanctity. However, I don't think that we can discount the apparent fact of mages simply being able to handwave problems out of existence as appealing toward players who want to have active characters who contribute to stories beyond simply standing around in the dale and dithering the day away - not to mention the amount of color and flavor a mage can add even in that circumstance that a more mundane character would lack. Compounding this with the borderline 'anything-you-can-do-I-can-do-better' nature of magic as a whole, you get a veritable perfect storm of flavor and function that mundane classes are incapable of competing with - and, unless we want to bring fighters to the point where they're literally cleaving moutnains in half with the swing of their blades, I doubt there's really much to be done about that.
Before this thread gets derailed - I would like to note that I make no remarks on mechanical diceplay balance. Frankly I consider that irrelevant to this topic, though I welcome you to open your own thread with musings about what numbers being changed on a character sheet will make you more likely to roleplay a farmboy with a giant metal stick as opposed to a literal god amongst men who bends reality to his whim and summons dominated succubi and angels to engage in raucous three-ways on the weekends. That being as it is, though, I'd love to hear contrasting opinions and I would love to be proven wrong.