The point is Tash we need strength in depth... you've flagged he's not a great player but there's nothing wrong with replacing him with better.
Personally I think he is genuinely woeful and get's exposed time and time again.
He might be a nice guy with a cool haircut but I want an ugly CB who will come in and do us a job if our first choice CB's are out.
Given we will never have a large squad he will most likely have to play a stretch of games.. which is where he will come up short.
How many teams have great cover in their defence though?
Man City -
Kompany and Lescott first choice - Savic and K.Toure cover. Hardly brilliant.
Man Utd - Vidic (Injury prone) Ferdinand - Smalling and Jones - Fair enough, good.
Arsenal - Mertersaker, Vermalen, Koscielny, Djourou...
Chelski - Terry, Luiz, Cahill, Ivanovic.
After these teams you can see that most good teams have maybe 3 quality options then average cover.
Williamson is good enough to be a 4/5th choice IMO, especially since we're not in that top bracket. If you look at the other sides that will be pushing in the top 10 next season, their squad depth isn't that great.
We wont be in Europe next season which means he'll get a cup games and maybe a few PL games at the most. I say keep him.