The admission requirements are probably not different from a student looking to do an MBA straight after college. They accept only juniors into this programme, I suspect, because they recognise that there is often uncertainty about what to do after graduating, and so many who are thinking of going into professional tracks instead focus on what they can do right away- Law/Medicine/etc. You can also study business right away, but few people have an understanding of what this entails, and even if they do, some may prefer to get a few years of work experience under their belt.
If that describes your situation, then you should go ahead and apply. The only thing to bear in mind is that HBS is notoriously very unforgiving of reapplicants, so unless you truly feel ready, not getting into the 2+2 programme may hurt your chances there later on. Then again there are so many great business schools that you'll still have many others to choose from...
it's for non-traditional applicants (mostly science majors) who would not apply for b-schools otherwise. it's a pure business strategy of HBS to expand target pool.