I've always been a fan of dynamic level design. I personally like it even in MP. The only Source level I released to the public had this for CS:S. My dynamic design for MP was to cut off some paths until different number of players were on the server. Thus the same level can accommodate different crowd sizes. So I don't think that MP should be ruled out of the discussion. A common example of levels that are kind of dynamic in MP are Battlefield levels that have different boundaries or points depending on the player size (albeit set at server setup time).
The main challenge when doing interactively dynamic levels in MP is that many players are not very "aware" of the environment. So my attempts at educating with signs that changed based on available routes was not successful because I would hear players exclaim "Why isn't this path open anymore?" or another scenario of "I didn't know they could get behind me here!" So it takes some educating the players--especially the loud and self-important ones that get angry at things that surprise them. When playing and I pointed out the signs, players might respond with either "STFU!" or "There are signs?".
But for me, these concepts are things that I find interesting both as a player and as a designer. I personally enjoy the stress of unknowns in playing, so I like that kind of thing.