the disparities in density and dynamics are always rewarding if you do them well as a group. i think a lot of people forget background/foreground in those situations (especially horn players and guitar players). my current frame of mind is foreground/mid-ground/background/no-ground (lead/counterpoint/accompaniment/tacet); you don't always need to play or be "interesting"—you can leave it to other folks.
Yeah, I've been fortunate to work with a number of visionary musicians -- e.g., Ken Vandermark, Chris Jonas, Randy McKean -- for whom foreground/mid-ground/background is not only one of the most important objectives, sometimes it's the
only objective. And KV is especially interested in establishing Who Is Playing -- and therefore Who Is
Not Playing -- at any given moment, and it's often fascinating how simply defining that one parameter is all it takes to impart palpable and audible structure.
if you're in improv land, it's not always easy to remember to *turn stuff off*. (i've messed up more than a few improvs by letting my loops go on too damn long).
I will proudly (almost to the point of arrogantly) claim that my gut nearly always encourages me to SHUT UP far sooner than most of the musicians I perform with. Whether that aesthetic/self-discipline applies to the loops I've generated is another story.

But I will say that in (most of) the instances where I
have let a loop go on too damn long, it's always a
really freakin' quiet loop. I was heavily influenced by Charles Ives, who would often write a piece where a line was intentionally buried, and then was only eventually revealed after every other instrument had called it quits.
do you ever use loops as motifs—using them early, turning them off and then reintroducing them later to create form?
Oh, hellz yeah. As someone who considers himself more a composer than an improvisor, I'm
always doing what I can to impart form. Recurring motifs FTW!
I'm pretty sure there are some bands who have a guy or a gal whose main job is that, or something like that. Portishead, maybe? I think there was at least one post punk band that did that as well.
Martin Swope with Mission Of Burma was kind of the archetype of this sort of thing...or, this sort of thing in
that sort of thing, specifically.