not much useful, IF u pvp, since u just spam, only useful thing in pvp, would be to know the other ones shield, since 2-3 attacks would drop any char's hp
on the other hand in pve, for example DF, if ur a segnale, it would be awsome to know ALL hp's, imagine that u got a friend that its in another team, would be cool to know his hp, so that u can help him as well, this would be useful to all other chars to calculate how much damage the mobs do...
still it wouldn't be a tool that would help u own or anything, simply would help ya out a bit
but i think its hard, since the program would have to access, the server or the others pc, since there is no real option to see other's hp, it would mean to hack either the server or the ppl's PC,
if what i said its true, then it would open a door to REAL hacking, making it a bad tool...chars would be stolen a lot more often..etc
anyway that was just an idea of how it would work, and its consequences...