Useless/Situational Abilities

Returning player here, haven't played for 6 years by now, after WoW it's so overwhelming to see so many abilities on classes with no "borrowed power" systems, but after I looked up a few guides I noticed that some abilities are not used at all, so my question is - Are some abilities technically "useless" for certain specs (for example Commando Gunnery rotation doesn't seem to use any grenade abilities (Mortar Volley/Plasma/Sticky grenade), why? Are they not optimal or worse on Gunnery Commando?)

Edit: Forgot to add that I'm talking PvE only, PvP is not really my cup of tea in Target-Based MMOs. (and it's obvious that the more choices you have & know how to use them = better odds in PvP).