I sometimes wonder if there isn't a real bias that budget travellers have against roller bags. I wonder if part of the "backpack all the way" mentality is so we budget travellers can differentiate ourselves from the damn tourists with their roller bags and tacky clothes.
Seeing as I travel alot of work, I have about 7 different options to choose from.
The full-on backpack (with zippered on day pack) I reserve for pleasure trips abroad that involve a lot of moving around - South America, Costa Rica and (upcoming) Tanzania. Part of it is because when I get off at a bus station I have no idea where I am going to stay, so I know I could be wandering around from hostel to hostel figuring out what has room and is worth my $5.
I have a couple different day packs for lighter or shorter trips. These I usually use for day hikes and or weekends out of town that won't require any nice clothes.
The big roller bag for business trips of more than a week or two or trips to resorts where I will need pool wear, daytime wear and dinner wear and want it neat and unwrinkled.
The small roller bag is for week-long business trips. Perfectly fits 5 days worth of work clothes, 1 day of casual clothes, 1-2 days of work-out clothes plus a pair of runners (if I really cram it in). This is the bag I use the most because most of my travel is around North America. It has probably flown a good 1/2 million miles with me over the past 7 years, but is getting a little worn out now. Probably time for a new one.
Finally I have a duffle bag that I could easily fit 3 bodies in. I save this for trips where I need to transport bodies, or when moving from one long-term accomodation to another (if I am on business for a while in one place).