It saves you about 6% of fuel, assuming the launch location is on the equator, is suitable for roads and can handle a lot of noise / avelanches. Getting into space is the easy bit (energy-wise). Getting up to orbital speed takes much more fuel and energy.

The problem is that now you have to transport everything near and up the mountain, which costs a ton in logistics (getting a road up a mountain that can handle a entire assembled rocket isn't cheap, nor easy). Most launches have delta-v to spare because satellites are usually nowhere near the rocket's maximum payload weight.

tl;dr it can save 6% in a ideal situation, but the extra cost and logistical problems aren't worth it.