This seems to be a controversial subject. I'm not sure what I'll loose if I decide not to renew. It seems a bit expensive. 
What are the advantages other than future factory support?

Go through 3rd party vendors like everyplugin.
 
The upgrades are only worthwhile if you bought a bundle.

Is there any way to tell if there are upgraded versions?  The functionality doesn't seem to exist in Waves Central. 

wup (if that is what you are meaning) is optional, i have paid the wup on some bundles that i own when there was a benifit to me, ie additional plugins added to the bundle  which bundle owners get for free. otherwise there really isn't any real need to wup, unless of course there is an issue with a particular plugin, something which i myself have not come across and i own around 90% of the available waves plugs.
 
you can see if there are upgraded versions or if your version is behind by checking in your account at the waves site, again, there really isn't a need to wup, unless there is a benefit to you. if you don't wup your plugins will still be available and work as they always have regardless.
 
myself, i will wup everything every other year just to be current (although it probably doesn't matter, and or as i said if something is added to a bundle i own which i will get for free as owner of said bundle) that's my take anyway.

To the op , i upgrade only when benefical for me , understand here only when they added enoght great additional plugins to mercury for me to jump , alwayz dealing here with wavesuniverse for all my waves deal , super fast and pro , exellent support and commincation ..
If you own a bundle that has zero addition other time , the only beneficial thing to update would be only if your system need the last version for compatibility

None of my Waves stuff has ever been updated. I don't see me ever paying the WUP. My WUP is more than I paid for all my Waves plugs combined. Really a concept that doesn't fit in with their frequent blow-out pricing. 

No WUP, no sir.

I always check to see if any additions plugins has been added to my bundles and if so, then I may upgrade, but other than that, I don't upgrade. 
 
I own it, it's on my computer and it works.

Zo
... the only beneficial thing to update would be only if your system need the last version for compatibility

you don't need WUP to install the latest version. reinstalling plugs using WAVES Central always gives you the latest version. Unfortunately, there is no way inside WAVES Central to tell if there are newer versions available so you need to remember last time you installed it and check the release news

I had a heap of Waves things, WUP'ed them once, nothing was new, sold almost everything eventually. I'm down to two Waves plugs now and will NEVER pay for WUP. If you have bundles, it may be worth it, though. Lovely plugs, just don't like the company that much.