hmm for some reason the post I made here earlier didn't make it.
But anyway, if you have an enemy portal starting with the second wave and each successive wave your entire TEAM will gain FULL creepwave xp AND gold as if you had farmed it.
Creep waves spawn every 31 seconds so, let's say 2 full locks' worth and 2 tp scrolls of investment to keep the portal flag on your team. Obviously no one died getting the portal nor keeping it 'cause that's just stupid 
The enemy team will have one guy farming the portal, the other guy farming the mana side creep wave, and then their third farming the hp side so each person on their team is getting full gold and xp from one creepwave. My team will have one guy not farming at all (he's keeping the portal flag), and 2 guys on the hp flag, sharing the creep wave. So one guy is getting no farming xp or gold, and the other two are splitting it.
But, my entire TEAM got 2 full creepwaves of xp because I held the portal flag for 3 creepwaves. This means that the hp guys got 4.5 creepwaves each (
.5 + 2*1), and the mana guy got 2*1 = 2 creepwaves so my team managed to get 11 creepwaves worth of expierence and gold. Everyone on the enemy team got 3*1 creepwaves so a total of 9 creepwaves. The two guys /sharing a lane/ will outlevel their /entire team/ by 50%. Plus, holding a gold mine denies them additional gold and grants you extra war score.
I don't blame you for makin the choices you do because you weren't aware of that mechanic. But if you can hold a portal flag for 2 full creepwaves, you will certainly benefit. You will /not/ benefit by holding it only 1 full creepwave because your team doesn't get the XP bonus until the second creepwave. So that would be a stupid thing to do.
If you can safely snag and lock a portal so you get two creepwaves out of it without dying... it is worth it.