Why? There is no law against using othe rpeople's licks, so it's not as if there are likely to be copyright issues. Also, the likelyhood of a) two people from this site being close enough to one another that people are going to hear the same licks from others or b)that both bands will become famous enough that everyone will hear them is so unlikely that i't's not really worth thinking about. Anyway, as useful as riffs can be, they're never going to be thee whole song (if it's good) and no two guitarists are ever going to interpret the tab the same wayy. I think the pros (helping others to learn new ideas) far outweigh the cons.