The problem IS the disproportionate amount of effort it takes to defend portals. It's quite easy to get a portal flag and keep it permalocked but getting it back is damn hard.
Well from my point of view its completely opposite. Why ? It's very easy:
1) You CAN teleport to your flag to defend it. You CAN'T teleport to enemy flag to take it.
2) Portal flags are near your crystal. If you need to regen after fight, just go to crystal, it's just few meters. Opponent need to return to his base, so basically run through whole map, or use tp scroll. If he use tp scroll, remember his tp scroll will have 30 sec cooldown, so it's good time for you / your allies to push.
3) There are towers near portals. Ok that's not a big problem late game, but if you defend towers well (using tp scrolls and universal gadgets), they are actually usefull (but still should be buffed imo). 2 towers supporting 1 defender, even late game makes a difference.
4) You can lock your own portal. Lock it, and try to strike meantime. Attacker can't take locked flag. Beside, even when you can't fight 1 vs 1 against opponent (eg. he has much better gear), you can tp to your portal and lock it, before he get even close to this portal ! It's all about how good you are controlling moves of your enemies.
5) When you own portal, there are creeps supporting you. Of course minotaurs and archers won't help you much, but priests/angels/catapults and especially giants helps you A LOT. Sometimes it's even hard to take unprotected portal, when giants come out.
6) Portal flags are just great to make a trap (others can join defender and strike attacker from behind). I killed houndrets of players who thought it's a great idea to take my portal flag
Well they were wrong
Most of the time, it's like free money, and money always gives you advantage. So capturing enemy portals is very risky, if you fight against experienced players.
BTW: If you don't know how to use the advantage from defending (in this case), you can't say something needs to be nurfed. That's why we have dozens topics here, something is overpowered and need to be nurfed. If someone is beaten by a better player, he just look for the reasons why it was unfair, instead of THINK how to improve himself !