For Diaspora, they are, at least for me. Any "classic" FS2 mod can be played without them, but for more advanced flight models, they're the key to successful evasion. Kicking in roll and sidethrust at the same time will decrease your hit probability to almost zero, and with some skill, luck and a proper ship, you can even shoot and kill something when doing that (Heavy Raider is a nice ship for this, since it doesn't spin too fast and has six well centered, very fast and very damaging cannons with plenty of ammo). Not advised in most situations because of inaccuracy, but can get you out of a sticky situation if you know what you're doing. Not to mention that sidethrust helps with landings and strafing capships.