Question about evolving and double characters

Hey guys, me again. Sorry if the title is misleading. Basically I have two different questions but since they are somehow related to each other I'll just post in one thread.^^

1. Evolving
Short question, is it always recommended to evolve characters? I ask because sometimes during the East Blue quests you need unvevolved versions (such as Mihawk, Fullbody or Morgan). For that very reason I'm keeping the latter two unevolved. However the question is if this is the only situation in which you better not evolve your units or are there other cases in which I should think twice before getting rid of a unit that I might need later on.

2. Doube characters (different types)
With this I mean things like having Log Luffy AND Gear 3rd Luffy, GP Usopp and Impact Usopp etc. Now there are a lot of character units that are pretty much useless. These are best used on their better versions to get some sockets out of it. However my question is appart from the standards like those that I just listed aswell as Lion Song Zoro and Asura Zoro, are there any other characters that have more then one usefull unit?

The reason why I ask this question is because I actually really like to collect named characters characters from the manga (even guys like Minokoala in the recent fortnight). Just having one version of a character is enough for me. However if there is units that are really really usefull in order to beat tough opponents then of course I would keep several different versions of said character. Same thing counts for evolving. As always, I'd be happy for some infos.

1. Evolving is almost never bad
The only bad things are 
1) for quests you need sometimes unevolved versions (but you can do the quest still later, after the fortnight comes back, or like Mihawk you can use him also as friend captain...) 
2) the special changes and gets worse or changes totally, GP Usopp, Mirage Nami or Streaming Wolf Zoro are some examples (so this units you shouldnt evolve...)
3) the worst case is if you get Sengoku xD units like Marco would be really stong in a Sengoku team, if they arent evolved. After evolving they will be too expensive, so Sengoku doesnt boost them (but then you got also the question, how high is the chance to get Sengoku...?)
right now I dont know an other reason to not evolve a unit, so Im always evolving mine

Thanks will keep that in mind.