Not all of Wilson's books are meant as pure science books. On Human Nature and Biophilia, for instance. He mostly puts forth hypotheses that are scientifically testable, but that doesn't mean he doesn't speculate. In Future he does have a tendency to use a rapid-fire approach, but all of his statistics are good statistics so far as I can tell. The projections are another question. There are, of course, different models that make different predictions of future outcomes based on the continuance of contemporary ecological practices. Plus, even something as basic as the ecological footprint is not exactly uncontentious. There are different ways of calculating the footprint that will give you conflicting accounts of whether or not current population levels and growth rates are sustainable, both long-term and short-term.
Wilson writes as an authority, but it should be remembered that he really isn't. He is primarily an entomologist and behavioral biologist specializing in social behavior. His knowledge of ecology is certainly well above that of a layperson, but ecology is not an area he has carried out a whole lot of research in (I have no idea whether he's ever taught an ecology class at Harvard). His writing is genuine and is the writing of a man that deeply cares about the subject he writes on and the readers that he writes for, but it's important to remember that the topics he writes on are all topics that are at the very edge of developments in the biological sciences.
The decrease in biodiversity over the last hundred years or so certainly is startling. The impact that a continuance of current extinction rates will have on man isn't really know. To be sure, there likely have been genes lost that might have been useful to us in some way, and the ecosystem disruption has been dramatic enough to badly hurt many indigenous peoples that relied on them, but all in all, mankind itself has been the better for all of our expansion. Wilson, however, is not only concerned with the impact the loss of biodiversity has on man, but the impact is has on the entire biosphere. He genuinely cares for the species that no longer exist and for the species that are withering on the brink of oblivion. He also appeals to the rest of us to care for our fellow earth-dwellers in this manner, even developing the hypothesis in Biophilia that this love of other species might be genetically programmed into us by evolution. Whether or not that hypothesis is true is a matter for further investigation, and he knows as much. It probably seems counterintuitive to a lot of people (it certainly does to me). But whether or not the love of biodiversity and other species is a part of our progamming has no impact on the ethical question of whether or not we should have this love. Ultimately, if you answer that question in the affirmative, then you will side with Wilson and this book might become an important work for you. If you answer in the negative, then you might very well say "so what, mankind will get along." Wilson also tries to make the case that mankind will not get along so easily if this continues, but that case is not solid. He is correct to say that nature provides for free many services that are being obliterated and must now be provided at a cost, but if the worldwide capitalist revolution continues to increase wealth on this planet, it might very well be that no one is the worse off for it, economically at least. The question of whether or not we should up our efforts to save the biodiversity we still have may very well come down to an aesthetic appeal if it is to have any impact on a person that does not care for other species. Do you prefer to live in a natural world, and do you think future generations should continue to have that option, or are you okay with the increasingly synthetic world that we build ourselves, discarding the prosaic remnants of Darwin's grand idea as we barrel along? It just might come down to that.