You don’t have to replenish your account in waves, since you can pay a commission for coins of any Bitcoin sport for any tracing or order. Bitcoin sports can be bought for Bitcoin, Ethereum, Waves and Dollar by replenishing the dollar account from a bank card.
I think having some waves to pay for fees is necessary because in support section, there is page that said: " Waves is the main currency of the whole Wavesplatform. So, before creating an order or outgoing transfer, please make sure that you have enough Waves. ". Anyway, 1 waves is only cost around two dollars at current time, and this amount to cover fees for many order on the platform.