Yes, absolutely worth it with giants and catas.
If your team successful locks, and relocks without losing control of an enemy portal, no amount of money, items, artifacts, levels of enemy xp will be able to take out 2 giants per wave x4 portals x2 waves per capture. Yes it's worth it. You win. That's how you win the game.
A competent team will lock a flag, and never let it get unlocked..... ever. Just keep relocking it, locks cast faster than a portal can be stolen, and portals wont change sides if you are present while an enemy is present. As long as you have a TP to get there, a lock to cap with, and you don't cast the lock until after they interrupt you (if the player even knows how to intterupt
)then you can cap infinitely. That's what you're supposed to do.
That's like saying you play in the NBA, 10 seconds left, you have possession, coach gives you a play to run. You decide to not run the play because you have an open shot at the basket from a position that you hit 100% of the time, and if you hit it you win the game. Typically, not running the play a coach gave is awful, it ruins the team, you're not supposed to do it (just like dying alot and feeding the team). But if you do it, and win the game, all that matters is the W not that you disobeyed your coach and didn't follow the plan.
You'll lose favor points in the end.... but in my games I'm that guy who permalocks and the other team curses at. I already have like 20k+ favor points anyway, it's irrelevant. All that matters is the win, and permalocking with giants and cata wins games hands down.
EDIT:
ALSO READ THIS THREAD TO SEE THE EFFECTS OF CAPPING AN ENEMY PORTAL ON YOUR GOLD AND XP:
http://forums.demigodthegame.com/364951
If what I said above didn't convince you that link will. And if you did agree with what I said above, then the obvious answer to OP question becomes so obvious you must face palm yourself.