gain 4 racial HD instead of each class level?
Outsiders gain full BAB, d8 HD, best saves and 8+int skills per level, with 4 Racial HD instead of a class level you could totally rock without any class levels at all. Qualifying for PrCes would be very easy and with the exception of some dips (i.e. Barbarian 1) you could use them for great mundane builds.
Btw I do think even some casters could consider it, because a wizard 7/outsider 4 has all that a wizard 7 has + better saves + more bab + more skills + one more feat (that would be Practised Spellcaster, so you end up at CL 11). It would be a trade on CL and useful stuff vs. actual spell progression, which is quite OK for some builds (IIRC you could just Outsider 4/wizard 2 for very much skills and you'd qualify for Abjurant Champion, so at ECL 4 you'd have a BAB +6, in the end you get a CL boost of 2 in this build and more than full BAB while still having the spell progression of a sorcerer. Powerful in my opinion, gives that you have much more skills than a rogue)
edit: Oh, and tanks would be really easy to make. If you take an Outsider and grab Improved Toughness, you gain 4d8+16+4xCon hp per level and +2 to all saves per level. With that BAB you only need Wrathful Healing and Power Attack to have a decent in-combat heal. A couple of more things and you got your tank complete.