[U]ltimately, the American inclusion would mean that the Man Booker is voluntarily ending its status as an arbiter of English literature—a canon with a longer and decidedly different cultural and political history than American lit, which the Booker itself played a role in transforming. Considering how quickly the Booker earned that arbiter status, it seems to me a pity to give up the prospect of continuing it.

Tim Parks is also opposed:

[T]he Man Booker Prize is simply following a trend which tends to weaken ties between writers and their national communities. … [Considering American novels] would reinforce the illusion that Britain and the US share a common culture. Above all it would contribute to a growing feeling that the author is an international entertainer rather than an artist involved in a home community with a literary tradition. In fact the rise of the international award goes hand in hand with the decline of the novel as a serious influence in national debate, or a medium where the native language might be mined and renewed. To top it all, the Americans, basking in a global power that confers cultural self-sufficiency, would be underwhelmed. No American author will prefer the Booker to the Pulitzer.

Leo Robson finds the hand-wringing unncessary:

Certainly the prospect, for a British writer, of a whole new category of competition, whatever the nationality, will not be welcome. But to imagine that Booker juries will be engulfed by a wave of American genius is to exhibit an odd inversion of Cultural Cringe, whereby the former empire becomes falsely convinced that, compared with those of a successful former colony, its own achievements are piffling, irrelevant, and drab.

Robert McCrum approves of the decision and puts it in context:

[I]n the evolution of English-language culture in the contemporary world, this is a small but significant milestone, a recognition that you cannot lay claim to being “most important literary award in the English-speaking world” and exclude the American literary tradition. …

Here’s the bottom line. Booker is a longstanding literary trophy. But no amount of longevity can disguise its essential character: it’s a lottery; a sweepstake. It has only a coincidental and fortuitous relationship with literary excellence. As Julian Barnes put it (in a phrase that’s almost obligatory to quote in these discussions), Booker and the other prizes are simply “posh bingo”.