It's also worth mentioning, roles tend to be fluid, but remember your main job, because chances are if you're on Gen D and you go outside help on Base D (or repair,) the generator defence will be weaker.
Similarly, if you're on flag D, knowing when to be a chaser and when to turn around and hump it back to the flagstand because the enemy capper is getting too far away for you to get back to fill your flag D role when the flag does come back is important. Ok, the capper might only have 20 points left, but let the chasers get him if you're meant to be on the flag.
Even a capper can use the flexabilty to assist in the role of base defence while you wait for your chasers to reel in the enemy capper and return your flag. Nothing's set in stone, but remember what your role is MEANT to be.
If you win a CTF match, your whole team is awarded more more XP than the losing team for the same kills and accolades, so it's in everyone's interest the team works together to mesh these roles and tactics into an unbeatable multi-tiered defence, and an overwhelming, strongly co-operative offence. Plus it's more fun to win.