As the poster above, melee normally will use their lb on raids when its "safe" and "required".

I play melee a lot as of late, i normally play blinding mage (rdm), and before we reach LB3 bar i always pay attention to the state of the battle, if i see we're doing well and there's no risk of a wipe or something going bad quite soon, i'll use LB on cd to try to stretch for a LB1 later in the same fight. IF i see the healers are struggling or people are standing too much in bad, i will delay LB3 application till i can see the fight has gone long enough to safely use it.

You say you see a lot of melee doing it .. its very common for melee to do so in certain dungeon bosses (Specially final bosses) and in other AR bosses, but not all cases.

There's certain fights where one doesnt normally use LB3 sans healer, to leave the fully lb2 for the trash that will come next in AR. Best examples i can come with are the iron guard bots on Ridorana Lighthouse, the large bots after 1st boss in PvP murder bunker, and some may save LB2 for 2P clones after Hanz/Gret or after Red Girl in Tower.