YOU get half the gold of a kill for a turret, but your TEAM gets 2.5 times as much gold for the turret as that one kill (150g x 5 = 750g for a turret). Turrets > kills for gold unless you're killing someone who's on a long killstreak and get massive bonus gold for them. There's plenty of incentive for defending turrets: map control, lane stability and denying the enemy team global gold being the main reasons.
As for Yi, Xin and Rammus, I have to admit I didn't really find any of them all that problematic in Dominion simply because the champ I used most often can deal with them 1v1 pretty well, at least under a friendly turret: Ryze. Xin jumps in on me? Snare, back off a step, nuke him with the turret's help. Yi rushing in with Highlander? Q-W him on his way in, unload entire combo on him under turret, he either runs away without even getting a hit in on me or continues trying to kill me and dies. Rammus powerballing in to taunt me? Snare, nuke, walk back under turret once his taunt wears off if he catches up after the snare ends. He doesn't do all that much damage by himself.
Unfortunately it's the super-bursty and/or stealthy people that mess Ryze up since they can just avoid his combos by disappearing in the middle and/or out-burst him under a turret, though I've found even the stealthers are stompable if I run Exhaust/Garrison instead of Promote/Garrison. Exhaust/snaring that Akali that just jumped in on me under my turret and garrisoning the turret to make it fire faster is pretty dang effective (which is why Exhaust is basically the Flash of Dominion: everybody takes it).