I've always though this "337 time doesn't count" thing is a bit over the top. Pick any random person here with good (500+) multi-command time in the book - how much of that time (outside simulated conditions in training, of course) was flown with assymetric thrust? Generally zero to not much. My point being, I see no real difference in the "quality of experience" between a C337 and a "normal" twin of similar performance, such as a Duchess. You have to manage two engines, and when one goes bang after takeoff you've got a poorly performing aircraft that will kill you if not handled properly.
The only real difference is you shouldn't find yourself upside down in the 337. However, you've got a couple of other little things to worry about that can bring you unstuck under stress (such as identifying the failed engine from instruments alone, managing gear retraction so you don't fall out of the sky when the speedbrake gear doors pop out, etc.). In fact, I'm pretty confident in saying I'd rather have an EFATO in a Duchess than a C337 - if the rear engine fails, anyway.
Granted, I wouldn't feel like applying for an airline job with 95% of my twin time on the 337 - but I see no logical reason why it shouldn't be counted as "real" twin time - the placement of the engines on the airframe has no bearing on anything when they're all working fine.