I agree that the band did a great job in choosing the right vocalist for most all of their songs. I also am one of those that usually likes the original version of a song best. However, there are a few such as the ones I mentioned above that I think would have worked equally as well (not better) with one of the other guys singing it. After seeing him perform it, I absolutely think Glenn singing lead on LITF would have worked just as well. I feel the same way about Already Gone. After hearing Don do it, I think that song would have worked just as well with him singing lead.
I agree with Maleah that I will always love Randy's version of TITTL best, mainly because of the ending and the high notes that Randy hits are perfect. However, I have to say that the non-acoustic version of the song that Glenn did on the PTO tour this summer was amazing (much better than the F1 version, IMO). Except for the high notes at the end, which Glenn could never hit, it was very close to the original version.
I also believe Glenn would do a beautiful job with ICTYW, but I'm glad he doesn't do it since it is Tim's signature song with the band (at least for now).