Just having extras-devel enabled is dangerous, you could accidently install unstable updates over stable extras software.

From the Extras-devel wiki page:
Basically this means (to me) as developer that it's perfectly ok to upload stuff to extras-devel just to make deploying it for testing on my _own_ hardware easier.

Now, imagine someone has a stable version of my software, fooblamatic, version 1.6.2, installed from extras, and also has the extras-devel repository enabled.

I find some cool undocumented feature in the API which would make fooblamatic foobla twice as fast. (It might also make the device explode.) I upload a new version 1.6.3, using this feature, to extras-devel, in order to deploy it on my testing farm of 13 N900's. Unfortunately, certain someone who has the stable version, accidently also updates the new version to his phone among updates to other software...