Honestly I can't give you a nice short reason why I personally left out 1 & 2, but since I was never going to do all of them, I ran a few SDN searches and tried to see what most people say about the NBME's. If I remember correctly (and it's been a while) the fact that the NBME's consist of retired USMLE Q's means that the first forms are the 'oldest of the old', and could POSSIBLY be less accurate in assessing one's progress. However, it could be that they change the Q's if, for example, a drug get's taken off the market, etc. Another thing -> I think that few people do form 1 (at least on SDN) and as such it is difficult to know much about it, whilst forms 4-6 get done quite a lot, which means you can see that person X got a 241 on Form 4 and a 240 on his test, which is reassuring...
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If you are going to do ALL the NBMEs and both the UWSAs, then sure, I'll do 1 & 2. But if I had to start leaving forms out, those are the one's I'll leave out first!