I am talking about unlockables in multiplayer games, which is what the term 'pay 2 win' usually refers to, since there is someone doing the winning. There is no 'finishing' these games.
If you are talking about 'pay 2 win' in singleplayer games (I guess we should call this 'pay 2 finish'), then yes that's pretty lame, but I can't think of many examples of this. I believe when Dead Space 3 came out people were complaining about being able to buy health kits and such, which ruined the atmosphere of the game? As long as things like this are not obtrusive though, I don't see why having optional paid-for add-ons in singleplayer games should bother anyone. Just don't buy them if you don't want to see them. It's not like the NPCs of the game are going to be buying them and using them to kill and teabag you.
I am defending microtransactions as a whole, because I do not believe they are universally a bad thing, because of the examples of their positive use that I have given you.
Yes, there have been instances of poor usage of microtransactions, such as maybe Battlefront II recently (I don't know for sure, I haven't played it). This does not mean they are always bad.
If games are to become increasingly complex, with larger budgets and teams of people working on them, which they are, then the people making them need to find even more effective ways to make money from them. Microtransactions are one way of doing this, and they can be implemented without being obtrusive, as has been demonstrated by companies like Ubisoft, Valve and Blizzard.