It's a good signing for all involved.
TNA needs new faces who can work towards the top of the card. If they're to survive, they need to keep building new stars. The biggest problem with TNA is that a lot of their talent is made up of guys who came from the WWE system. So signing a guy like Mike Bennett gives TNA a fresh talent who they can help mould.
The move also helps out ROH who have become a little stagnant and need to free up spots for other wrestlers on their card.
Finally, the move gives Bennett an opportunity to improve himself as a worker and break through the plateau he found himself in ROH. Bennett was a guy Cornette had earmarked for big things, but got lost in the shuffle following Jim's departure. ROH tried everything, but it didn't work out. After 5 years in the company, it was time for a change and Bennett has an opportunity to break out.
This is a huge show coming up for TNA. They have been utterly irrelevant for two years now and need to give fans a reason to tune in. With Lucha Underground gaining traction and ROH recording some of their best business in their history, TNA has got to pull out all the stops.