Here are a few good points to explain why I think that the idea that people shouldn't cosplay finished series a little silly. One reason is that some characters' true motives aren't revealed until the end (I'm thinking of Pandora Hearts, Black Butler and Fullmetal Alchemist to name a few) and this often changes whether a cosplayer likes the character or not. Why should a person be barred from cosplaying a character that they started liking late in the series?
Also consider all the very popular cosplays to finished series: Until recently Sailor Moon was a finished series (they are releasing a new one now, but still) and look at how many people cosplay that. That's not to mention Fullmetal Alchemist that went through that twice, Death Note and Final Fantasy VII. Also on a slightly different note people who cosplay anime that have finished but the manga hasn't (I'm specifically thinking along the lines of Black Butler, where it's pretty obvious which continuity it is in many cases).
Although, especially with very recently ended shows/games/movies you should be careful about any spoilers for those who have not finished the series yet that could be made apparent through your cosplay. That being said, that shouldn't be too big a problem with MLP.