While I do think variety is good but for me all those times I've never had Scizor (except once recently) and really I knew just about nothing about it except it was 4 times weak to fire when I had it I pretty much had to learn about it 's stats while training with it..and it was hard,also the trade and trade back thing is what gets to me because I thought initially that's what would happen when trading with someone else but that wasn't quite what happened...so for me apparently trading with myself was the only way to go with those evolutions and it's kind of hard (for lack of a better word right now) two at the same time,also the reason why I have never had a Slowking..I've had a few Slowbros but never a Slowking and I've had the kings rock too just no one trade with..and I didn't think of trading with myself them..and also I've only had Machamp twice,so I think if you have someone you trade and trade back (if you want if back of course) with then it's excellent but if you don't have anyone to trade with...then it's not particularly fun well at least for me anyway

What I usually do is put up the pokemon for trade and say that i'm looking to get the same exact pokemon, thats how I got my Gengar