Not really. You'd get access to a wider variety of rims and tires, you'll get a tad smoother ride over small obstacles and you'll get a little higher gearing. The price for this is plenty of brake hassle, unless you're building the wheels up with disc brakes, the cost of another wheel set, and a slightly higher ride height.

Narrow slicks that can take a decent pressure are already available in 26", so there's not much gain there.
Starting with a MTB you're already so far behind the purebred road bikes that going to deep section aero rims isn't much point. The higher gearing isn't much of an issue either unless you can honestly say that you're currently limited by spinning out in your top gear.