The linkages are still competitive, and they are by no means only with "bottom-tier" medschools. For example, here at Columbia we have many linkages, even one with Brown University, which is certainly not bottom tier!
I disagree that a postbacc is less competitive than undergrad. In fact, I think it's a great deal more competitive because the postbaccs don't "change their majors" or decide to go into research instead; we're here for one reason: to get into medical school. Most postbaccs with linkages are also two years, not one (I don't know of any that are 1 year, unless you mean an SMP, which isn't a postbacc). Many undergrads also have linkage programs--usually with their medschool. Even those without medschools often have linkage programs with other schools. At my undergrad, Providence College, premeds could apply to Brown Med at the end of their sophomore year and know where they were going by their junior year! I knew of many premeds who did this, and I thought it was excellent. (although this particular program was open only to RI residents)
The key is doing well in your postbacc courses, which is difficult because you have to take the core premed classes in pairs rather than spread out over 4 years. Also you don't have any easy upperlevel classes (and lets face it, every hard science major out there did better in p-chem then they did in general chem; once you take the premeds out of the equation, those courses get a million times easier) to raise your BCMP. So it's much harder, IMHO.
I always wonder why it is that practically every undergrad premed majors in biology. If you ever end up in medschool, that is going to be totally repetitive--why study the same thing twice? It's as if every premed thinks that to study physiology in medschool you needed to have physiology 101 as a premed. What sense is there in this?
Oh, and by the way, even those postbaccs who don't gain admission through a linkage still end up getting in somewhere if they finish. I know many many postbaccs who are going onto top ten schools next year; I even know of one Columbia post-bacc who got a full ride to UPenn!
As you can tell, I'm very pro-postbacc; I think this is definitely the way to go about beginning a career in medicine. The main drawback is the extra cost, because a postbacc is two more years tuition. However in the end, you are definitely better rounded, better prepared, and far more certain of what you want to do because you've already explored your alternatives. I should also say that postbaccs are, in general, older and more mature than their 21 year old ugrad counterparts. In general (and this is not always the case) a 26 year old interviews better than a 21 year old, just like a 21 year old interviews better than a 16 year old. That's just reality, like it or not.
I don't think many postbaccs consider it a "backdoor" into medschool. Every postbacc I know has far more volunteer and research experience than their undergrad counterparts. Many have advanced degrees or have worked in healthcare. If you're an undergrad considering going into medicine, do yourself a favor and don't major in the sciences! Major in literature or history and do your premed sequence concurrently. You only push your chance of gaining an acceptance down when you decide to major in biology or chemistry. Don't believe me? Take a look at the MSAR.
My $.02
