Mr. Head's semi departure from Buffy this year is a prep job for

1.) The character seperation neccessary for Buffy and the Scoobys to become real adults, with no adult supervision (because, err..they're not kids anymore....but that means I'm not either, which is scary...anyway..).
2.) The fact that Anthony Stewart Head is going to get his own show on ye olde BBC, still in the character of Giles. The show hasn't gotten kickstarted just yet, according to the BBC's site, but you can find out some info here:

Apparently, Joss's ambition is to do an "adult" show, a Scooby gang for adults. There are already plans to premiere it on BBC-America at the same time as in the UK, so us Yank fans of Mr. Head will get their fix too.