Having ridden mountain bikes for years, I was always under the impression that square taper cranks would not last as long as splined cranks, since the square hole expands after a while. However, nearly everyone who rides road/fixed/single speed seems to use square taper crank sets. Could this be because of the reduced stress of road riding compared to mountain biking, or am I just misinformed? Do any of you guys and gals use splined cranks/bb's? Is there any advantage to square vs splined or vice-versa? I just don't want to be replacing cranks often.

the mounting hole in square taper cranks only wear out every time you install it.
There's no reason for the square hole to wear out if the crank is properly installed.

ISIS and octalink cranks don't have as many selections for chainline because the spindle length selection is minimal.

The hole in square taper cranks does not expand on its own, unless the cranks are not installed tight enough. Wear only occurs when you pull and then re-installed the cranks, and with modern sealed BB's that should not be very often. All my old rigid MTB's have square taper cranks and open bearing BB's that required frequent maintenance, and those cranks have withstood multiple removal/replacement without wearing out (the spindle draws flush with the bolt).