Hybrids are basically a hack, not something that seems worthy of a core book.
Just want to address this - I think Hybrids were just brilliant in a very specific way - they're AD&D-style multi-classing that for the most part works and is balanced. Except for the wacky armor where as an example, an Avenger|Battlemind who went Con/Wis would start out with an AC of say 11 unless they spent feats or Hybrid Talent on AC.
4e's really the only edition that solved the "I want to be 50% X, 50% Y, start at 1st level with some capabilities in each because, you know, I grew up with X for a Mom and Y for a Dad." problem that didn't involve weird XP and leveling issues.