When should I use rich black?
What kind of black should I use when designing for CMYK print? explains what rich black is, but when should it be used?
If it gives a "blacker black", why not use it for all black elements in a CMYK job (body text/line art/tints)? Would there be any downsides?
This is more of a community wiki sort of question, but the obvious problem is that with super-fine lines (small type sizes etc), your type may at the very least become fuzzy because of the dot screens used to compose CMYK.
At the worst, the plates are misaligned slightly and you get color halos.
I have seen both in print.