Beneficial to the US auto workers who go jobs at the plants, beneficial to the local economies and governments who benefit from employed workers in their districts, beneficial to Honda and Toyota buyers, and beneficial to Honda and Toyota themselves.
If these plants had not been built, Toyotas and Hondas would have been limited in their imports, thus driving their prices higher. Making their competitors' products more expensive does not provide US auto manufacturers with the incentive to produce better or less expensive cars- I would think it has the opposite effect. It's just a nice break from competition and reason for them to keep doing business as usual.
Can't speak to any incentives which may have been offered to Honda or Toyota, although I suspect they were more motivated by pressure from the VER, and I doubt Mercedes' position of a few years ago is comparable - they are in completely different industry segments.
I think incentives are useful but local governments have to be careful. I live near the town that gave all sorts of tax breaks and incentives to the reborn Excelsior-Henderson Motorcycle company, which was using the resurgent popularity of Harley-type cruisers to start up again. Short story, bad management sunk the company (by the looks of the manufacturing plant they built, they spent a great deal of their money on architecture and lawncare- it was the most palatial manufacturing plant I've ever seen) and left the local governments holding the bag.
Back to free trade, I'm beginning to doubt there really IS such a thing as free trade.