Some of the good leveling systems would do much more business if their start-up costs weren't so high. Several hundred dollars just for the TLS gun and a bucket of caps, then add 50 cents or so per square foot to each job for the straps. Unless you're really knocking it out every day it doesn't pay.
There are some things about the LASH system I don't care for, like having the push the spikes in by hand, and the fact that they don't work on thinner tile, but it is a cost-effective leveling system.
This could be said of any system, but the biggest advantage to me is when setting tile on shower walls, they will hold everything in place and reduce the chance of a tile sliding down. This is especially important when setting tile on a 45 degree, when spacers aren't always effective.