Ewer on a blm shouldn't mess up their rotation unless they do something stupid, but on the other hand it serves no benefit either; we are still going to make the same number of casts then refresh ourselves up to full mp. We benefit much more from arrow, as we have infinite resources and quicker casts allow for more mobility, our inherent weakness.

Bole and balance are fairly obvious.
As Jack said, a random spear can make some CDs come out of alignment. A spread spear pre-pull can be pretty decent, as all CDs will come back up a little earlier.
Healers take priority for Ewer, probably then BRD( or MCH) if they have a song up, and lastly SMN (tho they can usually manage their own mp, unless they've been helping out with rezzes)
Spire on anyone that's been rezzed, or a tank, or then anyone that's running low

Spear/Ewer/and Spire are most likely to be your RR fodder, as they are more situational