There are a few different schools of thought here.
Argument 1: It doesn't matter.
You could easily make the argument that the Flyers aren't winning a Cup this year anyway, so really, what does it matter? I find that argument to be a little silly considering the Flyers *did* give up some value here. A fourth-round pick is valuable, and if the Flyers make the playoffs, this turns into a third-round pick -- yes, even if they get swept in the first round, which is about the same as missing the playoffs in my book.
There's risk for the Flyers, but I'm okay with that risk. Ultimately, you can't exclude the sentimentality factor here, the fact that Simon Gagne tshirts and Simon Gagne jerseys will be purchased by Flyers fans between now and the end of the season. That might not matter on the ice, but it matters. It matters both in terms of general excitement about the team in a year that hasn't seen much excitement and in terms of actual dollars coming into the organization.
Argument 2: Gagne hurts the Flyers.
I don't particularly understand this argument either, but it's there. You can argue that he's not worth the draft pick, but the chances we miss that draft pick when all is said and done are relatively slim. You can argue that he'll take a roster spot away from a younger player, but if he's not contributing, it's not hard to just sit him.
The Kings did it, and then they traded him. See how that works?
Ultimately, the Flyers didn't give up a roster player for a guy who has a chance to be a valuable piece. If he hurts the team when he plays, sit him. If he doesn't, play him. Really pretty simple.
Argument 3: Gagne provides the Flyers with on-ice value.
Do you think Gagne's less valuable than Harry Zolnierczyk or hell, even Mike Knuble or Ruslan Fedotenko? He may have been a healthy scratch in Los Angeles, but that's a different situation and you can't really compare across teams like that. There's risk, but I'd bet on Gagne being more valuable than at least four or five forwards currently on the roster.
Gagne is still a capable two-way player in the NHL who can play solid minutes. He's not a 40 goal scorer anymore or even a 20 goal scorer, but he's not 40 years old and useless either. He's still fast, he's still strong on both sides of the puck, he can play more than 15 minutes a night and he can contribute on the power play.
Once you add in all that veteran leadership nonsense, Stanley Cup experience (!) (lols), and of course, familiarity with the organization, Gagne has all the tools to contribute as a member of the Flyers this season. It might not be a trade for a defenseman or The Deal The Team Needs To Succeed™ but that doesn't preclude it from being a good trade.