You also lose out on a lot of the networking, which is also extremely important. In fact, I would argue that that may be the most important part of the B-school experience.
Nevertheless, a 1-year MBA may definitely make sense to some people, especially those people who already have a good network and/or already know exactly what it is they are going to do. This is especially so for those people who are being sponsored by their employer. In such cases, you obviously don't need the summer internship because you're going to go back to your employer anyway. The networking becomes less important too because you should already have a strong network at your job.
MIT Sloan runs the Sloan Fellows program, which is Sloan's version of an Executive MBA program, but can be treated as a de-facto 1-year MBA program.
http://mitsloan.mit.edu/fellows/overview.php
Stanford also runs a Sloan Fellow's program, which is Stanford's version of an executive master's program. You won't get an MBA, but instead get a MS in Management, which is a comparabe degree.
http://www.gsb.stanford.edu/sloan/
However, most of the top 1-year MBA programs are located overseas, especially in Europe. For example, top notch European business schools such as INSEAD, IMD, Impresa, IESE, Oxford, and Cambridge (but notably not the London Business School) are 1-year MBA programs.
I think these Euro schools are an excellent deal for those who want an international career. Some of these schools in particular are very good at certain niches. INSEAD, for example, is extremely successful in getting its graduates into McKinsey and other major consulting firms.