There's no reason to keep them. Even if Poetics can be converted to Law, the rate would be so low that it's just not worth the trouble to hold out. Even if you had like 2000 Poetics, it will probably be worth at most 2 entry end-game dungeon runs at level 60. If you need something now, get it. If not, you might as well not bother with the effort of earning those Poetics.

Personally I'm just spending everything to have a full set of 130 (well except a ring) for my two main roles (tank & healer), as I plan to level up DRK and AST in 3.0 anyway, and this little boost at 50 would be a good breather for then.