It's a design decision. It may be more flexible to use DSPs to transcode, if you need to transcode G.729 calls to UCCX (for instance).

If you are just transcoding to allow G.729 to use the SW CFB, it might be better to just define HW CFBs instead.

If you need more than 8 legs on a bridge, then HW CFB can't do it, so suggests SW CFB. If you use a lot of SW CFB, you need to watch CPU utilization.