In my hazy recollection, there was another, rather pedestrian reason Apple didn't go for BeOS: it had almost no infrastructure for printing. The Mac's niche was prepress and desktop publishing (remember that phrase?), and BeOS could barely print and had no color management.
(Though I could be totally wrong on this, and welcome a correction.)
I also read a story about how BeOS DPx (developer previews) lacked decent printing support and this was another reason why Apple chose NeXT. The irony is that Apple had to redo the NeXT printing stack anyhow, as did BeOS in R4, and they both ended up using CUPS. Also another reason was lack of x86 support, which forced BeInc to quickly rush out a x86 port in R3.0. Intel were so impressed by the x86 performance, that they ended up investing $4M into BeInc.
(Though I could be totally wrong on this, and welcome a correction.)