After making this comment, I decided to do some research and I just wanted to further clarify some things. Since there is a leaderboard for Prestiges, buying tons of Cubits, selling them, and Prestiging a bunch of times is pay-to-win, however, it does not break the Eula, because as I mentioned earlier, you get no gameplay advantage from doing it. This would also apply to the Cubits leaderboard itself.
It can be argued that buying ranks do provide a tactical advantage through things like /kit giving potions and enderpearls or /speed, however, I do not believe it does because the items can still be obtained in-game for very fair prices and /speed does not affect Challenge mode, which is where a lot of the competition on Parkour is.
As far as I know, I don't think the Parkour server on ManaCube is pay-to-win, however, if you think I am wrong, please tell me! I'd love to have a discussion about this. Also, bear in mind I only play Parkour, so I can't speak on the other game modes and if they are pay-to-win our not.
Cheers!