I don't think an omnibus has ever made me rush, or has ever forced me to dumbly and absentmindedly plow through a series. When omnibuses are done right, they contain the entire story/run they claim to in a giant, dope package littered with extras, that invites you to either take your time or plow through--but that's more the type of reader you are.
Let's say you pick up 10 floppies in a week, how many of us get home, plop down on the crapper and rip through 5 of those bad boys right-quick? The retention rate at that point is probably even worse than say, plowing through 5 issues of Planetary because it's super engaging and captivating, because you're juggling 5 different comics, 5 different continuities, 5 different worlds.
Also, if I tip my bookshelf over on an intruder or just some asshole who's shit-talking Frank Quitely, it'd totally crush them. That's another positive of omnibuses.