If I have tokens left over, I use the chest again and again. It depends on the other bonuses you get. Not every chest is worthwhile. I've already got a few Tupolevs and good accelerations. On my main account, I have 400 tokens for good direct offers. I use everything above that with the 6 token chests. It's worth it, even if there's no legendary artifact. I do the same with my other accounts and have occasionally received legendary artifacts for 6 tokens. From my point of view, it's not necessarily a game of chance if you look at the total possible rewards and evaluate them for yourself. That's the good thing about Dominations, it's strategy, even with the 6 token chest and if you play the game properly, you have to work out your strategy and for some it may or may not be worth it.