Endgame raiding and High-end arena pvp are two completely different games, and making cross comparisons in gear between the two is retarded.
PvPers cannot take their 450 resilience gear into Black Temple, and raiders can't use their mp5 and spirit gear in competitive arena pvp. They just won't succeed. No 2400+ team is gonna take some raider with no resilience into an arena, and no BT guild is gonna take a Vengeful pvper to down Illidan, unless they've got him on farm and said pvper is paying thousands of gold for a cool trinket or something.
The two different gameplay styles are mutually exclusive at end-game, let's compare them back and forth.
Arena PvP is about encouraging fairness and skill, and its gear system reinforces that. Any gap in gear is a sharp disadvantage and is not conducive to an atmosphere where the most skilled and dedicated players succeed, while the crappy ones don't. A fantastic rogue can just start out in arenas and will get stomped until he can gather the gear. Once he does, however, he'll be able to prove his worth and rise to the top. Good for him.
Scrub players can lose arena games and collect their gear, slowly. But scrubs in epics are still scrubs, and they'll still be stomped by better players in equal gear. They'll still never succeed, whether or not they have purples. Or is the game about collecting gear for you? Because for high-end arena-goers, it's not. It's about being the best. The gear is just a necessary step.
End-game raiding is about progression. Seeing and beating new and more powerful monsters. It takes dedication, attention, gold, and also some very specialized gear and tactics. And the gear system supports that progression. You collect blues from 5-mans, then you do Kara, then you work your way up through Gruul and SSC, then The Eye, and you end up in Black Temple, having seen and beaten all the content until then. And you do ZA at some point if you wish.
PvP gear being upgraded every season? Do you really think 2200+ teams care? Hell, they probably think it just a petty annoyance to have to upgrade their gear. But in reality, it's necessary. If PvP gear remained stagnant while PvE gear continued to be upgraded every time a new raid was implemented, sooner or later we'd have raiders destroying PvPers, like before BC came out.
The PvP gear system works for and supports PvP. The PvE gear system works for and supports PvE. And since you need resilience for PvP and mp5 for PvE, you can't mix the two.
The system works, and it works for everyone. Some people like to fight AI, some like to fight other players. It's stupid to make people who want to fight players, fight AI first in order to be competitive.
So, raiders, the next time you see some scrub wearing full season 1 epics that you know they didn't work for, remember that they'll still never succeed in PvP, and resilience won't help in PvE. And if collecting gear is the point of the game for you, maybe you should try something else, because it doesn't seem like you're having fun anymore.