Indeed. You've basically got Demko and Dipietro nicely offset, to where Demko should be graduating the AHL just as Dipietro arrives. Give Dipietro 2+ years in the AHL and you should have 2+ years of some form of NHL experience for Demko, and a decent idea of whether he's going to be anything or not. ~3 years from now, you probably want to have another quality goaltending prospect ready to move into the AHL.
As far as i'm concerned, you basically always want to have some sort of quality "developmental" goaltending prospect simmering and learning in the AHL. If you don't have that, you're wasting an important development slot. That's a lot of utility to just throw away. You can have problems on the other end as well though, if you logjam things, because goaltending is a unique position where one guy is playing the whole game...and the backup isn't doing any developing that night. So it's really easy to overload the pipeline and have more prospects than you can effectively develop. It's a bit of a tightrope to walk, but if you're going to err...it seems better to err on the side of too many guys looking promising and not really having space for them all.
You just have to keep that pipeline flowing, because goaltenders are a slow-developing position where, you need to have the guy in the pipeline developing like half a decade before you actually need that starting goaltender in the NHL.
Especially these days, where you pretty much have to find two quality goaltenders to keep your starter optimally rested for a long season, especially if you have aspirations of a long post-season as well. Having a spare starting caliber young goaltender can be a really big advantage.