Quiet UAC can create some odd problems, most (bit not exclusively) because of registry virtualization. It also leaves installer run as elevated system user free hand and if installer gets it wrong it all goes to poopers. I had instances where I, as admin v0n wouldn't have permissions to write to files installed in v0n's own directory under, let's say, roaming profile because with Vista's bizarre implementation of what could have been just a simple "sudo" type elevation, any member of Administrators group can prevent other administrator from access. Effectively, with UAC, you have to remember you are never User elevated to Super User, but instead you are User elevated to Admin User in kingdom where just about everyone is Admin too.
So why is silent UAC so bad? Because it presumes continous elevated access unless it encounters something it cannot read or write to, and that leads to complete corruption of permission in some cases - you get to the point where some part of your install program was run with "Run as Administator", installer switched off virtualization and then user, elevated or not, will not have access to that stuff. Sooner or later this leads to installations where recursive permissions are completely shot, links don't get added to your start menu, games with saves that can't be found from search in explorer, folders which can't be deleted to the end or progs which can't be uninstalled from "Programs and Features" because some important file has gone walkies. Sounds familiar?
The simple fact is - if you are power user, with a lot of apps, you either decide to run UAC with popups on, or go through your life without UAC. There is no viable inbetween.