It should not matter if that client supports other currencies too, as some "multicoin" clients too and also some web wallets like coinbase that allow you to accept Bitcoin and US dollars.
Of course, it does matter. Bitcoin XT client will NOT allow you to accept both Bitcoin and Bitcoin XT currencies if they both exist.
In the (unlikely) event that XT ever does replace Bitcoin, then all major clients will not allow you to accept Bitcoin if both Bitcoin and XT exist.