While I love Star Fox 64 with undying passion and will defend games on rails, one of the best parts about DQ8 was the beautiful world, and the grandness of that world would have been lost if I couldn't explore it at my leisure. Yes, the plot progression was linear, but if you're going to say that, then basically any game is. It doesn't matter how many optional things there are in GTA, there are a series of key missions you have to do, and your options of who to take missions from are limited. Similarly, why aren't we complaining about Zelda being too linear, then? The point is, it doesn't REALLY matter if it's on rails, but the freedom to discover things on your own, even if everyone would have eventually reached the same end point, is what makes certain games so much fun.
As they say, it's not the destination, but the journey that's important.
Personally, I'm more concerned that this game is going to be too focused on the sword play, and not on great characters (hands-down the best part of Dragon Quest).