This contradicts a clear historical trend in which Perl regex syntax and POSIX regex syntax (not the same) are being more widely adopted as time passes. It's a definite step backward.
This wasn't a deliberate decision on Apple's part to remove this particular functionality. Rather, they switched to a different implementation of grep (BSD instead of GNU) and it leaves out PCRE support by default. If word of this gets back to whoever made the change, it should be a simple matter to reenable it for future versions of OS X.