The key word to this discussion is "composing". Real Band provides me the backup band so I don't have to bring people in. (I don't like having people in my house.) All the plugins I really use are the ones that came with Sonar and Izotope Ozone, and I can use those in Real Band on the master bus. When I DO leave RB I go to Sonar but that is only after the RB stuff is saved to a stereo wav file and I am just tweaking it with Ozone.
I play almost nothing live anymore, and I define "composing" as the creation of the song. Once it is "composed", adding sweetening tracks are not "composing" anymore in my mind. It's already written and I am just filling it out. This is the reason I love RB. I can add live tracks just as easily in RB as Sonar. RB is ready to rehearse at a moment's notice when I am since it lives right in my house. It never shows up late and/or drunk, and I only had to pay it once. Great tool for moving stuff from napkins and envelopes and voice mails I sent myself into viable, tangible music.
I particularly appreciate the drums. I simply don't have hours in a day to be programming drum machines or snipping drum loops from the drum loops discs I have. Sitting there auditioning if "16th fill with snare and high toms" is the right loop of the 5000 to pick from. They are 2 second looks, but that is 10,000 seconds, or 166 hours, to sift through drum wav files. Let Real Drums do it, and I can add stuff by manual tap mode form my Roland drum machine later.