Two of my last three PIC projects used PIC12 parts: PIC12CE519 and PIC12F675 (the latter of which actually has the 14-bit core). These are both autonomous units rather than slaves, though their functions are admittedly very simple.
The third uses a PIC16C716, and is most definitely a slave device.
I've never actually used a PIC18. More complicated projects tend to get AVRs, though there is an HC908 project in the queue...
All micros (PIC or not) are chosen on that basis. Where I work, at least.
But PICs, AVRs, and HC908s have the advantage that we already have development tools for them.
And another potential upcoming project may use, umm, someone else's part, because 1) the micro is on the customer's "approved" list, and
2) the chip maker wants the design win bad enough that they'll, umm, work with us.Regards,
-=Dave