I get the argument, but it's a retrospective one only and that's the shortcoming. Yes, of course Tate was more valuable last year. He had a career year. Harvin had one also...for games missed. You can't make performance per dollar argument yet because Harvin's one year into his contract. You're quitting the Indy 500 after losing the first lap. If we're going to do apples and apples, let's pick their best seasons thus far and compare:
In Tate's best year, 2013, he had a total of 1571 yards, including receiving, rushing, kickoff and punt returns. Only 5 TDs, all on receiving. None on returns.
In Harvin's best year, 2011, he had 1832 yards, with 9 TDs, from a mix of receiving, rushing and kick returns.
I should note that in 2010 he actually had more cumulative yards because he had a freakish 933 return yards alone. His rookie year he had 1156 return yards. Both those years he had 40+ returns, and 24+ of them were for over 20 yards.
What's more, you can't simply look at player stats alone. Look at players around them. In the Super Bowl game, look what having Harvin on the field did to open up the game for guys like Kearse and Baldwin. He's so much more deadly in so many more ways.
I'm not dogging Tate, good player and he'll do well in Detroit I think, but for the very reason that the guys here will do better with Harvin playing: there's a #1 to take the heat, while the #2 pick the secondary apart with the holes created by the threat of that #1.