Sure, if the list of DPS were like this:
assassin rogue 1000 dps
fury warrior 999 dps
retribution paladin 997 dps
.
.
.
arcane mage 990 dps
Then you are correct, logically speaking, being in last place does not mean garbage. However, in reality, in the encounters that matter most, arcane is far behind the pack. So much so that the top parses of arcane are in line with average parses of everyone else.
What this means is that if your raid group is struggling to beat the final raid encounter, but getting very close time and again, then YOU BEING ARCANE is the reason why. Being arcane is a liability to your group. They would be better off with literally anyone else in the group.
In fact, plain average fire mage DPS is better than top performing arcane DPS, so if your group is getting close in the final fight, but not quite getting there then you not switching to fire is like purposefully tying a boat anchor to a drowning man.
Couple this with the fact that not only is arcane simply terrible in the last 2 fights, the most important fights, but they are bad in EVERY fight, and have been for this entire expansion and the ENTIRE EXPANSION BEFORE IT!
There’s no logic in that. Sure, nobody can expect to be at the top of the dps meter every single pull, but you shouldn’t be mandated to the bottom of every pull by your choice of class/spec.
— one more thing —
Arcane is DEMANDING AS FRIG! In order to get those top parses that are in line with everyone else’s average parses you have to play perfectly and get SUPER LUCKY! ONE TINY MISTAKE can crush your dps as arcane. Get CC’d in your EXTREMELY SHORT power phase… hang it up, you’re done. Anything that happens during your power phase, boss moves out of range, you have to move out of fire, you have to slow an add, you have to throw up your shield, you have to soak something, basically if you have to move for any reason then your DPS for the ENTIRE ENCOUNTER is going to be average at best, and average arcane parses are bottom barrel parses for everyone else.