[k0nkuzh0n]

I think the reason why it is suggested is because it can't hurt to do a hard reset and could possibly eliminate several mysterious causes of problems that happen so often in electronics.

It most likely roots from how memory works. if you "delete" a file from your hard drive, you don't erase anything, you just allow that location to be overwritten. I accidentally deleted my entire library of movies once on an separate partition (it was the only thing on it) . I was trying to wipe a flash drive and selected the wrong drive. I was able to recover every file through some program because the locations that contained all those files had not been overwritten yet (since it was only used as storage).

so "deleting" files when flashing roms may very well have fragments left over that could cause problems. unallocated ram is like the mysterious-beyond of computers and can cause problems if assumptions are made of it.

Hard reset I believe actually goes out and sets those important bits and bytes to ensure they are what they are suppose to be (no assumptions).