HI,
yesterday from that new little unuseful pokemon I got a 100% Ditto at a nice level... Does it have a sense to keep it or power it uo? The only interesting use I see considering that he becomes what his opponent is could be to use it in a raid against a boss that has a move which is also super effective against it... Like a SB Alakazam or a SB Gengar... Does it make sense?
It's not a terrible thing to use when the typing is right, but Ditto retains its own HP when it transforms. Since that's fairly weak, Ditto won't survive for long and typically falls behind other counters in general effectiveness.
I'd keep it though. Any 100% is worth holding on to, if only for collection purposes.
I'd keep it for sure, 100%'s are pretty neat collector's items.
Powering one up would be more of a luxury than provide an actual use. As Skywalkered mentioned, Ditto retains its base HP upon transformation which hurts its bulk significantly. From a practical standpoint you're better off just using your own version of whatever you're copying.
