Well, first DMs need to be online.
I for one sometimes connect on the evenings, but don't always have the time to make a quest.
I think week-ends are best for me, since i have some time - but then again, maybe even sundays, not the best day for many people
And yes, DMs can do all you say. Last fun i had was put rewards in Scorn so that people had prizes (free apartment extender, stat potions, stuff like that)
Remember too that setting up quests can take time, especially if you do something complicated. And it's more rewarding for a DM if there are many people participating, which leads to the issue of the level of the quest - not all players have the same level, thus some things may be too hard for some, or too easy for others.