This is an opinion - frequently development libraries, header files, etc. are dependent on kernel versions, glibc, etc. More so than binaries themselves. At the very least, a rolling release will change your kernel and glibc. Which means unless your header packages change in tandem, they might cause incompatibility problems.
It is less of a technical issue and more of a workflow one - would you want to handle the explosion of support requests similar to "I have mysql 5.1 but my PHP client does not connect to the DB.... oh crap, I forgot I have mysql 5.0 headers. heyyy, what gave you the right to upgrade my mysql ?"
If you go with the notion that disk space is cheap (and indeed packaging headers and dev libraries would be not too expensive in terms of disk space ), there is really no real reason to separate them both.