Mertis are wonderful whether you want to level up a second (or third, fourth, etc.) job, or try to make your main job really effective at end game events (ie HNMs and gods).
As a RDM main, I maxed out my enfeebling magic giving me a base enfeebling skill of + 15, as well as putting several points in Wind accuracy (which aid in landing silences and gravity). When Kirin time rolls around, I have very little trouble landing Gravity on him over and over again.
After a while, when it became apparent that my RDM skills were more than adequate, I started putting merits in other categories. % crit hit rate is one of the most popular in the "Other" category, because the only other desirable traits in that area are + or - enmity, which really messes things up if you're a tank who wants to try mage, or vice versa.
When I started levelling THF, just putting a single merit point in Dagger gave me access to Viper Bite at lvl 32 (as opposed to lvl 33, which is the normal level), allowing me to shock and awe the newbies with my damage output... at least during the very rare times they were able to line up properly for a SATA. ^^