Experimental is an all or nothing proposition. With a PPA system it should be possible to keep most of your system on stable, and opt into newer versions of software on a case by case basis.
Doesn't apt-pinning solve that? Set a pin that never installs from Experimental unless manually specified, or only install that one package from Experimental?
Just a note for the uninitiated: when you pin from Stable you just gave up the guarantees that may have been the reason you were on Stable in the first place. This is an area where RTFM is in order:
Well yes, but all the warnings and cautions in that link apply equally to PPAs - the security implications are pretty similar. A novice using PPAs will be in the same boat, though admittedly PPAs are a little easier to use.
Oh I was being a little tangential and pointing out that changing packages on Stable to an extent defeats the purpose of it. I just hope they don't start releasing PPAs that'll help people achieve worst-of-both-worlds systems that combine the outdatedness of Stable and the bugginess of Experimental.
Packages equivalent to named long-term development branches of upstreams, without packaging the result under a separate name (thus confusing dependent packages)?
I already mentioned some of relevant points in the comment you replied to:
1) compartimentalized, not a big, global dumping ground
2) same builders network as unstable/testing
3) no need for a reupload to push to unstable/testing
We already have Experimental.