A properly built NHL team won't have useless players on their 4th line. Unfortunately, the Flyers usually do.
There's nothing wrong with starting prospects on the 4th line. Some prospects won't need that, but a lot will need that time. It gives them a proper opportunity to learn the NHL game, something you can't do playing top minutes in the AHL. Once they seem comfortable, you can move them up the line up.
The Flyers unfortunately don't do this. They would rather sign useless veterans like Deslauriers, and give them regular icetime over any young guys with potential. They also fail to know when to push a guy up the line up, like they did with Couturier.
4th line players need to bring something more to the team, and no, being good in the locker room isn't it. The Deslauriers-types are useless and a complete waste of a roster spot. Face-off ability, PKing, good forechecker, speed, solid defensively...these are something they need to be effective in the role, which is usually something a young prospect can bring and not what we are used to seeing out there on our 4th line.