I did a bit of searching on the matter, but all I could find was that it's just not allowed for some reason, when it worked just fine in 2.2.
I'm interested in finding-out what the technical reason is for disallowing persistent database connections in the current release.
I recall in 2.2 there was an issue with sess_write(), that resulted in writing too many Set-Cookie headers and it made our reverse-proxy error-out. Is it a related problem? Not sure if that's still the case, because I'm in no hurry to upgrade that project to 3.0 without the customer requesting it.
