Here is a list of players that made a name for themselves around the NFL as a Giant. This is also a list of players that left for free agency while playing at their highest level to instantly fall in the dumpster of washed-up players. Yes, these players were all talented, but its the organization and team around you that makes you the superstar you are. Not just your raw ability. Leaving one to argue; should a player stay in a good system that has given them success? Or leave for a big contract that they only deserved because the Giants' coaches put them in situations to be successful?
When it comes to wide receivers, pray for a good QB. No matter how awesome you are, you won't get the chance to display it if your QB sucks. Randy Moss sucked in Oakland, people thought he was done. Then links up with Brady and sets records. Deon Branch, great with Brady. Goes to Seattle and plays like a has-been. Goes back to Brady and re-makes a name for himself. Steve Smith, he breaks Giant records with Eli and then misses out on a 2nd ring because he wanted to go suck with Vick. (I'm sure many more examples could be made.)
Any of these Giant free agents better be wary of these trends. Success is far more important than any amount of money. Manningham and Jacobs will likely disappear from the NFL lime-light if they choose to leave. For their sake, lets hope they don't.
p.s. - It has already been a long enough road and struggle to get Manningham to finally play at the level we expect since his draft. Hopefully he doesnt let this 15-minutes of fame go to his head, because if he leaves and enters a new system with a worse QB, he'll just be starting all over again from the bottom.