The point of pre-ordering isn't much different from backing the game. You're essentially just funding current development on the promise that you will eventually get a completed game. There hasn't been any purely consumer oriented reason to pre-order games since digital distribution became ubiquitous.
I would say that there is an incentive to pre-order now that you can pre-load (at least on Steam). Maybe not a huge incentive, not like back when stores would run out of physical copies of a game, but if you have a slow connection it can make a difference. Besides, pre-orders aren't presented as "pre-order and get a complete game Eventually"; it's "pre-order and get a complete game on Insert Date Here." It does feel like it violates the spirit of the agreement a little. If you're considering whether or not to back a project, you're going to consider the developer's past work, and "Hey, remember the time they knowingly released a thoroughly borked game?" does not exactly bode well.
I think Deadfire will be a good game, perhaps excellent, once all the bugs are fixed, and that it's a lot easier for me to play Armchair Dev/Tester/Project Manager than it is for the good folks at Obisidian to actually do those things. They've worked their butts off (and are continuing to do so), and the parts of the game I have seen (that are not bugged) seem very nice. I think that's why I found this frustrating, but as far as life problems go, this is ultimately pretty minor.