Them problem is, don't rely on your turret to help you. Just keep hitting the enemies with anything that can stop their channel. The longer it takes for them to cap that point, the better off your team is as a whole. It's not about "winning" and defending, but buying time for your team.

I think defending creates some of the most memorable moments of this mode. I had PLENTY of moments where I felt that trying to defend a point was freaking useless especially if they had 1 enemy up on you, aka 2v1, 3v2,etc. But once end game comes around and your all tanky or dealing INSANE damage defending can be SUPER fun. I was laughing in the faces of anybody who tried to dislodge me as Leblanc yesterday.

It really depends who you're playing whether or not you can be good at defending. Heimerdinger is crazy good at defending control points with his turrets.

My best defenders so far were:
Maokai, who has his ult that reduces damage + 2 AoE attacks
Gragas, able to push the whole team off and a few AoE attacks
Yorick, one AoE and does quite a bit of damage + his ult that revives him to keep going after death, lol

Also, I've seen Mundos defend towers for an absurd amount of time too!

I main maokai and so i played him in dominion and it seems that his best use is as a defender. He's tough as nails, sapling toss is a long range cap interrupt, he can blow up the attacking creep waves fast, and he's got lots of cc to lock them into taking turret damage.

i also fought a swain who was impossible to kill under that turret... could not take the node while he was there.

all you have to do to stop the cap is hit them 0_o its not hard man

Held a turret as Ryze for 11 minutes until their entire team came rolling in.

I think your problem is mostly due to the best cappers being rather poor defenders and vice versa.

i find AP shaco with JITB one of the best absent defenders in the game, watch them hit a JITB and get feared while a turret wails on em for a bit