If you've got a Gunn-type situation it can avoid further unnecessary embarrassment and discomfort, granted, but you'd hope that kind of thing is rare. I think there are significant advantages of pre-announcing a long slate of films to go with the disadvantages you've mentioned (which I accept and make sense), but the key thing is that quality wins out at the end anyway and when you have the unprecedented foundation that the MCU has you don't have the same downside risks when you change/cancel/add last minute material. Audience trust is there and pretty much there regardless, so I see it almost as a free opportunity for awesome marketing. If the final films released are different to the slate but feature a consistent lineup of greats, the majority aren't going to care that film X was delayed by a year, film Y was introduced out of the blue and film Z was outright cancelled.
As long as the belief can be maintained that the announcement was made in good faith at the time it was made (and not subject to being completely torn up like we've seen with DCEU slate announcements), I think the advantages outweigh the upsides of not doing it. Plus if they were to stop doing these and still release a raft of great films, I would miss the ability to anticipate years in advance with a pre-planned structure that really allowed me to feel like I was part of the experience.
Edit: also I didn't really get that with AoU, and didn't feel that was the case on the Hype in the AoU threads during the build up. The main thing that let that experience down was Ultron's portrayal.