IMHO, azw3-only files should be avoided, since only a limited number of devices and apps support them. Also, master mobi files that contain both regular mobi and KF8 segments aren't significantly larger if you use the -c2 compression option with Kindlegen 2.4 and strip off the source with the
KF8 version of Kindlestrip.py.
If you need to use KF8, for example, for books that contain Hebrew or Arabic text, I'd recommend using Kindlegen 2.4 and
replacing the mobi section with a warning stating that this file requires a device or app with KF8 support. (That's what I did with my
English-Arabic .mobi file.)