牛人可以泛泛得写一篇无关痛痒的letter, which will not help. Even worse, they can just write this person is ok for a tie 2 PhD program but I would not recommend this person to a top 10 PhD program.需要的是牛推,like I have written letters for XXX and XXX went to MIT, I have written letters for YYY and YYY went to Wharton. This person is at least as good as XXX and YYY in bla bla bla and I believe he is highly capable for any top PhD program.