I'd like to contribute some thoughts about what "trade paperback" really means, and why they are preferred both by retailers and publishers (and certainly this author). It's not all about the size of the book, or the price. It's the trade terms.
Mass market paperbacks, as most know, were designed for a newsstand age where space was at a premium and consumer targeting was very poor. Publishers blasted out vast numbers of copies, printing several to sell one, just as they did with comics before comics shops. Retailers, many of whom did not really track their clientele or their interests, simply destroyed unsold copies, stripping their covers and sending them back for credit. It was the buckshot approach. The amount of waste was astonishing.
With the newsstand declining in prominence and the Waldenbooks of the world giving way to larger-footprint stores, mass market paperbacks were no longer as good a deal for the business. Because the discounts are tiny to account for all the wasted copies that have to be printed, the retailer makes very little per mass-market paperback -- not much of a reward for connecting a book with a customer. The trade paperback was the solution. They're larger and more durable, so when returnable copies go unsold, they're not stripped; they're returned as whole copies for resale elsewhere, just as hardcovers are. The discounts are thus better for retailers, explaining why you'll see Amazon offering better deals on them, percentage-wise, than for MMPBs, which usually don't go below 10% off. There are also trade paperbacks that are sold non-returnably (as almost 100% of the comics and graphic novels sold to comics shops are): these represent the best deal for publisher and retailer, as the discounts are usually higher. And, thus, the consumer, who's that much more likely to be offered a discount.
The mass-market paperback age isn't over, but the math works better for the higher-end format, especially now in the digital era: the print purchaser is much more likely than before to be someone interested in a higher quality, more durable physical package. The number of mass market paperbacks produced annually has collapsed relative to other formats, as has the number of rack spaces for them. And the MMPBs that remain are seeing changes to make them make more sense for publishers and retailers: the canon Star Wars mass market paperbacks are now all those $10 tall editions, which give the retailers more reason to stock and sell them.
In general, I support as many options as are feasible to offer, though I understand why all aren't out at the same time; the hardcover first run is to the paperback second-run as the theatrical release is to the discount second-run house. The early audiences are by definition the most motivated, and likeliest to be interested in the better formats. Maybe we'll see a TPB to MMPB model in this case, maybe not. We'll have to see what shakes out.