Just curious if people are bothering with twilight missions.. Is all the extra effort and time worth the reward?

I do them for the challenge. They are fun mix ups of normal missions (The one I'm doing right now has it's own layout, which is a nice mix up.). They also make good candidates for expeditions, to make doing the same level more fun on top of the new enemies.

As for reward, they are the only place (that I know of) to get quality umbracite while on NG. The Quality Umbracite can be used to temper better skills on to your gear, so it's pretty good to have.

Also twilight missions drops gear at your level regardless of mission level. So twilight version of first game mission drops lvl 122 items for me now. Can be quite handy.

How come no one is being summoned on my twilight mission? Are summons available?

I actually ran into that the other day. If the twilight mission has rotated out, you can't summon any visitors into it. Blue Graves still work though.