I'll be the first to admit that 5D can work, either as a profitable save or by giving LHO a chance to go wrong. However, RHO may have just a limit raise, and if partner has a strong hand, it's no cinch they have game. With this auction, I can't have a real hand. I'd hate to bid 4D and propel them to slam, much less let them know they need to bid a light game. This may make me sound like an LOL, but I don't think the upside of jamming them here is much better than the downside already mentioned.
Saves are a bad bet at imps.
Except when you gain imps. The point is that a preemptive 4D brings partner in on the decision. But if you pass or blast off to 5D, you're guessing. 3D is too constructive, and 5D is sort of like jumping out of a plane without knowing who packed your chute. We apparently have our choice of 7-card fits, and neither figures to play very well.