If the tank has no problem meeting the defense minimum and they have a fully gemmed and enchanted frost set, it will reduce their damage in the encounter to wear it.
There is more to taking damage the just total numbers; how you take it is much more important. Even if the tank took significantly more front damage damage than physical it would still be easy to heal as it is a constant predictable value. I heal at about 80% overhealing anyway...more frost damage would simply lower that number. It is the melee damage that you have no idea when it is not going to be dodged/parried/ect that provides any sort of challenge on the part of the healers.
I have never tried the rest of the raid in frost resist so have no opinion as to that but I can tell you as a healer tank in frost resist is somewhere between not worth it to makes it harder.
I've only done this in 10 man, with full frost resist gear on every player including myself as driud tank, where uncrittability is with talents.
Maybe I didnt need it as a tank, my health was always way up the whole tim anyway, so I dont know. But I had it in the bank, and thought I'd take it for a spin. We one-shotted him both times I've done the fight, so I'm not complaining. People's gear wasn't amazing; for many of us, it was the first time seeing Sapp in 10 or 25 man. Maybe it helped our healers conserve mana because I took a lot less damage,... I kept my eye on the raid and reminded ppl to move out of blizzard, which they did. Every single person got behind an ice block every single time. We had a lot more trouble on earlier bosses tbh.
We had the tank in normal tanking gear, and the dps and healers wore as much frost resist as we had.
For melee dps, if you wear the full frost resist crafted set and have a paladin aura up, you can ignore blizzards and be healed through. So the DPS you lose from the frost resist set you gain by not having to run away. It probably nets out even damage, but having the frost resist wins the tie breaker of being more tolerant of screwups.
I'm not sure for casters, but I know that our caster had a full resist set and lived through easy.
It was easy to tell who didn't have frost resist gear at the end of the fight. They were the ones being rezzed.
I am a caster who has never worn a piece of frost resist on this fight. I have never died in the fight yet. I move out of the blizzards when they hit no matter what I am casting or what buffs I have up.
But, I don't understand how anyone could suggest ignoring the blizzard, whether or not you gain DPS by doing so it isn't that hard to kill him before his enrage so the 3 sec you spend repositioning out of the blizzard is going to kill your chances of downing him? If anything repositioning is good practice for later when there is no resist gear its just a race to the enrage timer an you do need all that DPS you can get. Play smart not easy. Its easy to stand in an AoE with resist gear but its not smart.
What it comes down to is whether or not the main tank is wearing frost resist should not be the make or brake for this fight. Either way will get the job done. I prefer no resist gear on tank and some people prefer to have it.