As Kitru mentioned, end game PvE is all about specialization: especially if you want to be doing gear appropriate content. If you're running around in full Campaign or BH, spec isn't really important in a HM FP. Hell, those things have been soloed or 2 manned. Hell, I've tanked them almost naked on my Guardian (only belt and bracers for armour).

PvP Hybrids are usually interested in grabbing the Pull for positioning enemies and/or FiB for interrupting caps around/behind the node. Compared to Slow Time for PvE, it hits fewer targets, has a longer CD and doesn't apply debuffs or build HS stacks.