I don't know about you but when I think spellcaster I don't think about some guy/gal with a sword in one hand and a pistol in the other (one of the possible weapon sets of the mesmer). Honestly, whatever "mesmer-y" feel I had to the class died with the transition from GW1 to GW2. Right now the only thing I see as defining the class is the ability to create illusions and that can easily be added to dual pistols.
Would I like to see that OP? Yes. But I know it's never going to happen. Ever.
There's a few things you're missing here:
First, every profession was going to have a melee option. Sword happens to be that for the mesmer, for reasons we'll get into later.
Second, the impression I get from the mesmer's sword attack routine is that the sword itself isn't truly the weapon. In the autoattack chain, it's effectively serving as a conductor for the magic that the mesmer is using against the target. For Blurred Frenzy, it's a focus for creating an illusion in which the enemy is struck by multiple illusionary blades, where the mesmer may or may not be actually swinging the sword at all. Slot 4 is mechanically nearly identical to a sceptre skill, and the illusion-summoning skills are obviously illusion summoning skills. Essentially, mesmer sword is
a spellcaster weapon, just one that's intended for use in close quarters. Pistol use works similarly - the primary function of the offhand pistol is as a focus for generating the phantasm, while it can also serve to generate a spell that can interrupt multiple targets. It's questionable, however, if an actual, physical bullet actually plays a role in that, or if the offhand pistol is ever loaded.
Third, ArenaNet always had a duellist feel in mind for the mesmer. There's promotional artwork for the mesmer that predates the release of Prophecies showing a mesmer with a rapier in hand. Fencing and duelling fits the general mesmeric theme of misdirection, and there's a clear connection between illusion and deception and the feints and dodges of light swordplay. A single pistol, with the occasional powerful shot, also fits this theme... but historical formal pistol duels were about making the one shot that counted, not simply blazing away at your opponent. A pistol with a high rate of fire as a primary weapon fits neither this theme nor the theme of a primary spellcaster, unless something suitably extreme is done to show that it's not just a gunslinger with fancy bullets (a theme already well filled by the engineer).