When I finally acquired my first couple AlienBees, I did not buy any softboxes. I found that the Apollo Westcott 28" and the 50" softboxes worked (okay and well, respectively) with the B800's. These monoblock lights have umbrella attachment holes that would hold the umbrella shaft. I figured, "Great! I can still use my softboxes"
Here's the issue. These B800's generate heat. Lots of it especially if the modeling lamp is turned on. Enclosed in the softbox, the heat no place to go. With the 50" softbox, this isn't as big of a deal but with the 28" softbox there just isn't enough space.
Why haven't I bought real softboxes? Because the B800's are well-cooled and I'm mindful of using them inside the softboxes for prolonged periods of time. That being said, I have used them in the softboxes with the modeling lamps on with my normal firing rates before without a hitch. You gotta understand, the AlienBee B800's are workhorses. They seriously can take a beating. I don't abuse them, but I'm not "light" on them either. I'm sometimes firing them close to full-power once every 2-3 seconds for 100 frames at a time.
The other reason? I rarely use my softboxes. I opt for grids and my beauty dish more often and so I can't justify forking over more money for a proper softbox. That being said I do have the strip softbox with grid from AlienBees.
Hope this helps with the decisionmaking :)