I've decided the same. Maybe my initial bad luck plays a part in my thinking, not too sure. (not getting anything for anyone on my roster except for my 3 star chickens, which are only there until fusion starts and will then disappear from my vault). But it got me thinking about the in-game economics of this feature.
For an account like mine, and even moreso for newer ones, I think it actually makes no sense to start farming these things. F2P accounts will need loads of time to build a sufficient roster to justify going in there and playing roulette.
In more details, here's how i view it. There's basically 2 things you can do if you're mid game as far as Iron twins go. One, you're actually able to clear at least stage 11, which means you'll need to spend up to 120 energy on a daily basis, or 840 energy per week. Two, you're not quite at that level yet, and so you'll clear a lower level and not even get any soul essences for the runs you do. It will nevertheless cost you around 700 energy and this without getting decent rewards. Beginners who are stuck at level 6 or less shouldnt even bother.
840 energy per week, what does this mean? It means one third of a tournament not completed to full, so less epic books, or it means less chickens, which will impact the growth of your roster over the long haul, or it means less potions and you'll have a bunch of 6-stars without amulets and banners, or it means one less champion without masteries per month, etc. etc. etc.
So, it all comes down to cost vs. reward doesnt it. Spend 840 energy, and possibly have nothing useful to show for it, or forego this feature and focus on guaranteed growth. I choose the latter.