Bids generally have to be responded to within 100ms on these AdTech stacks and if you start to timeout they slow down bids being sent to you....which makes your bid manager less desirable by advertisers. You can probably use stream processing to build and update your models but I'd be surprised if you could handle the bid responses through the same mechanism.
For future consideration and delayed evaluation of course. I guess if you absolutely must have the most up-to-date information so you can make decisions on it RIGHT NOW that wouldn't work very well :|
Or would it? If your bids and stuff are also going through the event stream..