All subs have their uses.
Personally, I prefer /SMN in most situations. In events such as Salvage, /SMN really helps, especially at the start, when you don't have access to a lot of gear, especially if you have capped MP. Keeping Haste up on first floor is almost impossible without Auto Refresh, unless your BRD and RDM have access to magic, which isn't always the case until later on the same floor, or beginning of next floor. Besides, getting 12MP/tick in Salvage is sexy, although admittedly, Conserve MP while getting 11 MP/tick would be superior, but personally, /SMN > /BLM in this case, since it speeds up first few floors more then /BLM would.
In Dynamis, it really depends on how experienced your LS is, and what yoru "position" is. If people die a lot and you need to raise then, /BLM would be superior for Conserve MP. If you're in, say, a melee party that don't pull hate a lot, /SMN is superior. Hell, this is one of the situations where /BRD would be superior, in order to March them all. I've also experimented with /BRD in the Tank party for Ballad, in order to let the BRD throw Enmity songs on the tanks, Although fights are usually so short that it doesn't matter.
Ballista is one of the situations where /BRD is good, since Horde Lullaby have close to 100% accuracy in Ballista. However, when it comes to 1 vs 1'ing on WHM, I prefer /RDM or /NIN
On RDM, /BRD have its times. If you're not main healing in a party of, say, RDM WHM <insert 4 melees>, /BRD is decent in order to march the melees and ballad + refresh the WHM, and with Crimson Legs and Blink, pulling can still be done.
as for COR, most of the time RNG is a good sub for DD'ing. However, on some monsters, such as imps, I've come to like /BRD as a sub. A badly timed Amnesia can turn COR's in to gimp RNG's, and /BRD in this situation lets you keep March and Ballad up. Besides, /BRD also gives you something besides shooting to do in-between rolls. And I'm sure your healer will like Healer's Roll and 4 MP/tick Refresh, and an extra Haste for your melees isn't something to complain about.
All subs have their uses in different situations. You just have to know when one is more beneficial. And /SMN over /BLM (or the other way around) will rarely make or break your event. Personally, though, I prefer /SMN over /BLM.