If activating all of the extra features we're interested in is a problem, I'm sure the devs will tell us. So I don't think there's much point in not asking for it just because we're afraid of asking for too much. If we don't ask, we won't know.
So let's ignore any other feature requests we're interested in. Would YouTube embeddings be useful to us? I'd say yes, they'd be very useful. There will be questions about how to achieve certain graphical effects, and there's only so much words can do to explain what exact effect that is. Showing an actual video of it would be much more expressive. This applies to both questions and answers. So I'm fairly certain that we will regularly use YouTube videos to make our points... so why have people load YouTube and leave the site to watch the video, if they could just watch it right there in the browser?
It encourages questions of the kind how do i do this that do not always work out as valuable contribution.
I don't think so. If someone wants to ask a bad "how can I do X, plz give me teh codez" question, they'll do it with or without an embedded video. However, there could also be great questions requiring videos (think "I'm trying to achieve this diorama effect. I've attempted X, Y and Z, but I just can't the depth of field to do it's magic. What am I doing wrong?"), and making it easier for those kinds of questions seems like a great opportunity for this community.
What I'm saying is, we'll have to watch the quality of our questions either way. This feature is going to encourage "video-only questions" as much as having code blocks encourages "code-only answers". SE is a great platform for building high-quality content, and having YouTube embeds will in no way torpedo that mission.