Filppula is an excellent depth player. But he has truly failed to impress me as anything more than that this season. He's in that same category of guys like Ryder or Vrbata, who are nice players and all, but fail to engage that next level to make them indispensable.
So if the price is right, and since this team isn't overflowing with players of his caliber, you certainly keep him. Free agent options will be no better and I'm dubious the trade market is all that strong for a 2nd tier UFA having a soft season.
But with Tatar and Nyquist knocking at the door, you could make the argument that there are players ready to fill his shoes and if his contract demands make the team uncomfortable, exploring options would be savvy.