Ditto....power up if 100%?

Not at all a waste. Use the ditto caculator to see, but often your ditto will end up a higher CP than your opponent. Fantastic for making dragonites to use dragon moves on other dragonites.

It's highly impractical in my opinion. For one, Ditto's maximum stamina is 96. Even once you transform, Ditto keeps it's base stamina. So even if you have higher CP than the pokemon you're fighting, you'll always have less stamina unless that pokemon's stamina is below 96 which is really uncommon. Most defenders have much higher stamina.

For two, it can be fun to power up attackers but it's usually more important to power up defenders. The reason for this is because when you're dodging you can take out pokemon twice your combat power, thus maxing attackers isn't a high priority. Also, lower combat power attackers are better for prestige because you get more XP based on the gap between you and the defender you're attacking.

Finally, there are so many better options when it comes to attackers. Ditto isn't on the attackers tier list for a reason. Sure, it's a fun pokemon to mess around with but test a few fights and you'll realize how many pokemon can out perform it. Think about it, a same-type pokemon with less stamina than the defender its attacking.

? ? ; ? ?: Ditto's max stamina is 96 which is rather low and it can be out-performed by mediocre pokemon. It's value as an attacker compared to top tier pokemon is nearly useless. Only spend your dust on this if you have an affinity for maxing 100% IV pokemon. It's not practical in the slightest.

As a defender he's far too easily defeated. Not as bad as an attacker, but that stamina is a killer. I played around with lv29 84% IV Ditto I caught and decided he wasn't worth the dust to do anything with.

Not a waste, though max out it gives only 87 HP.