First off... I really don't understand all the Rhino hate. No they aren't great, but the time they buy me, the things they do for me... I find them almost mandatory even in my Renegade lists. Maybe I'm the only one playing guys with masses of indirect or long range but low end firepower?
The Rhino is pretty big to my army plans. I hate buying them, but I have 2 per list. Landraiders are hard... Started out in love with them, but just can't even make myself take one lately. That said back to the issue....
Zerkers with axes and a chainsword are good, but it's the mechanics of 8th that hurt them. I'm continually interrupted by assault armies with superior Command Points (all of my opponents seem to have more than me, but especially Nids and Orks).
So for me the breakdown of Zerker efficiency boils down to one round of close combat efficiency. I will get to go twice, then my other squad will almost always die (unless they're still hiding in a RHino/etc.)
After round 1 is over, typically that's it for the Zerkers too. So maximizing their output to me, means keeping them dirt cheap.
It is interesting to note though that I'd be better off with a Dark Apostle. I've just been in love with the idea of the Exalted since he dropped, but I guess I never gave the DA a shot in 8th.