I know this can be a can-o'-worms, but I only look to do a 650b conversion if a frame can't fit fat 700c rubber. At one point most of my riders were race-ish frames, most not fitting anything wider than 28mm, some maxing out at 25mm. That's where a 30mm+ 650b wheel can work wonders. Now I buy frames differently, and am very happy with 30mm+ 700c tires on frames that fit them.
YMMV
+1
Also you generally don't get the same type of geo you would get with frames that fit the wider tires. 2 fold reason. I don't think a 26" feels differently from a 650B, but the 650B makes a convenient size for conversion.