Your "mechanic" is totally incorrect there. The upper engine mount bushing has nothing to do with the two solid AC lines that are secured to sheet metal, not the engine. Take a look at it yourself if you doubt me.
There's been many ideas as to why they leak but most think they leak from the inside out as 134a's PAG oil and moisture becomes acidic and in time, it disolves through weak solder joints in the evaps. Pollen filters seems to slow the process down a bit but it's not a total cure either.

As for climates, it was originally rumored that some climates had more problems with failing evaps than others but I'm not sure if that's still true now.