While it's true that a format in ntfs is better than a conversion to ntfs, saying it's a "bad idea" is a bit of a stretch. NTFS still provides advantages in certain areas for those who need them and (especially security) and a conversion will work just fine. Wrong cluster size? Maybe - but maybe not - depends on what's on that partition / disk (file types & sizes). Higher level of fragmentation? Oh, I don't know. But even if that does happen so what? That's what disk optimizers / defraggers are for.
I just think that statement is a bit too general, that's all.