Actually, I wouldn't risk it without some sort of escape plan or a buddy to get this far away. The safest place to stand is 2-5 squares from the fighter, not 11-20. Ranged attacks be damned. If I had a player who was doing this every fight they'd end up in a really bad spot whenever there's lurkers about.
Likewise ray of frost isn't that bad, but it depends on the party. If you have a decent ranged lineup consider trying to catch enemy soldiers and brutes with it early so you can get a few rounds in on 'em before they can get into melee...
To each their own.
It's rare to have foes attack from behind, at least in published adventures, due to the fact that the PCs tend to wipe out everything more or less in a straight line. Generally, there is nothing left behind the PCs.
So, the foes would need to move 10+x squares to get to the Wizard (if the Wizard is 10 back from the Fighter). That is almost always a waste of an entire rounds worth of one or more enemy actions.
There are some Ranged 20 attacks in the various monster manuals, but that too is rare (< 20%).
This Wizard is handicapped somewhat. He's stuck with Ray of Frost and Burning Hands combined with a low AC. The only nice thing about this PC is that he has Improved Initiative and Scorching Burst, so he might be able to get one or two of those off early in an encounter before the foes can react.
If he stays close to the Defender as you suggest, he can easily be toasted by being too close to the enemies, and his Burning Hands might result in friendly fire on other PCs (it's a fairly large blast) if he tries to use it.
But if he is far away, he can use his Ray of Frost to slow up a foe that gets past the Defender (the successfully hit foe moves a max of 4 per round, the Wizard 6 per action), otherwise, use his long range Magic Missile or even Scorching Burst if he can catch two foes near another PC.
If he had different powers and feats, I would suggest different tactics. But with these eclectic options, this unusual set of tactics is better than more normal tactics with more well balanced Wizards.
As for Ray of Frost, it has three weaknesses if used as you suggest:
1) It targets Fort, so it affects most foes less often.
2) Even when it does affect foes, the foes will tend to be within 4 squares of some PC that they can charge, so the Slow has a minimal effect (or even be standing next to a PC, so no charge needed).
3) It does 1 or more fewer points of average damage per round than most other single target At Wills.
Every other single target At Will is better: Cloud of Daggers and Magic Missile do more damage (since Slow rarely is helpful), Illusory Ambush does the same damage but protects the PCs more, and Phantom Bolt does more damage and allows the Wizard to slide the foe (extremely useful in many circumstances).
All in all, Ray of Frost is the worst of the single target lot.