Take a look at the large number of bug reports you find here, and a decent proportion of them will be for just such an issue. It seems that the BG1 installer is somewhat buggy, and will skip files, and not report any problems, until the player's game starts crashing, or their tutu installer goes fubar. Rather than repeatedly sending people to the thread I listed to check to see what they are missing, I thought it would be easier to have the installer check.
Also, I'm not a big fan of the BP installation method, which is rather horrendous, and from what I've heard, the Tutu installer has a passing similiarity (or it did in the past). I figure anything that makes the mod more accessible is a good thing.