People keep talking about his "ability to stretch the field" but sheer speed and athleticism don't automatically equal deep threat (just look at Janis). Cook needs to prove he can consistently catch the football before he's a real deep threat. Not to mention that Jordy Nelson is the main guy to stretch defenses. Cook obviously has the potential to help stretch the field but if we're looking at what he's actually proven on the field, it's his blocking that will be the largest improvement over the Packers previous group of tight ends.
Jared Cook: 7 years in league, 273 receptions, 3,503 yards.
Jeff Janis: 2 years in league, 4 regular season catches, 95 yards.
Not really sure how you can compare these 2 players and what they can provide the Packer offense? The only thing they have in common is speed. Other then that, one is a WR with poor route running abilities and very little experience. The other is a 7 year vet. TE that has put up some good numbers during his career while playing with mediocre teams and QB's. I get your argument, but I think you are putting to much weight on the one knock on him, he can drop a pass or two. Sure, he is going to have to catch some balls to get the Defenses attention, but I don't think he is just the speedy decoy that Janis appears to be.
Given our cap situation next year, I think the Cook signing was perfect. Had the Packers not spent as much money signing their own, maybe they could have competed for Ladarius Green. However, in hindsight, I'm happy with Cook and the extra cap space of having him over Green, this will give us a bit more flexibility next year.