Wrong. I can stack up to 430 FR on my paladin, before applying the +50 resist pot. So 480 frost resist total, or about 73% reduction in damage.
However, that is because of the tanking set for Hydross the Unstable (frost boss) and it is all plate. You can find leather frost resist gear, but it will be random enchant material, so it's something you will have to look up on the AH and keep an eye out for.
There is a caveat: if you go full FR, you will be insanely OP against frost mages (I have not been beaten by one yet while I was wearing my frost resist set and buffed) but you will be mincemeat for any melee or other rdps classes.
If you go LW, you can get the +70 FR patch to your bracers, but that requires farming level 79s in Icecrown yourself (Easily done by a paladin, but not sure about the rogue).
You can also solo the escape from durnholde dungeon, and do some Black morass, for rep with Keepers of Time, for the +20 FR head enchant. Again, easily done by a paladin, not sure about rogue.
I spent about 10k on the materials for the 3 piece +frost resist tanking set, made by Blacksmiths, as well as the Frozen Eye, which is a +frost resist ring made by Jewelcrafters, as well as Pendant of Thawing, which is a +frost resist amulet made by Jewelcrafters.
So if you hate mages THAT BAD, go for it. You will not regret it one bit.
There is something strangely satisfying about killing frost mages so thoroughly, I couldn't resist the opportunity to obtain this set. No pun intended.
P.S. Even if they stack spell pen, they will be forgoing another important stat, and that will still allow you to kick some butt.
And for the record, I never use the FR set in arenas, since I have no idea what my opponents will be, and I cant equip armour while in the arena. So if you want FR as a dueling/BG set, for specific occasions, go for it.
P.P.S And don't go assuming that just because I have never lost to a frost mage in my FR gear, that it translates to -never- lost to a frost mage. Frost mages are extraordinarily over powered at this level, and because of the billions of ass-whuppings I've recieved at the hand of a particularly skilled frost mage, I have built this set.