I have not seen any arguments that have convinced me that money will even make a difference. The only difference I have seen is people with money will get 'Mechs faster. However, with time a player who does not invest money will be able to get the Battlemechs and then all players will be equal.
You may not be able to pay to win in a skill based game, but you can certainly pay to get an advantage.
Here is a hypothetical situation.
Say that the devs decided to implement so called premium Content.
They could add mechs that had more armor, and weapons/ammo that did more damage than their regular counterparts, and then make them available only to those who paid real money.
This would mean that those with the money would be able to buy them, while those that dont have the money would no longer have an equal chance of winning.
It may not be Pay to win, but it would certainly be Pay for an advantage, which in my opinion is very close to Pay to Win.