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.
In case you're the hidden owner of all major clients and you can speak on their behalf, let me tell you. The only major client that matters is Bitcoin Core. Get over it and forget about the Bitcoin XT altcoin.
Do you care to explain why the devs of clients would continue to have their clients remain compatible with a coin that (hypothetically) is not used?
I think whichever is superior will ultimately be adopted (assuming that XT coin ever comes into existence) and right now I believe that Bitcoin is superior, although it's max block size does need to increase. That doesn't mean that I won't change my mind in the future if something changes or if someone can make a strong enough argument.
I am not running XT right now and have no intention of doing so, although if a situation were to arrise that forced me to use it then I would be able to use it with my Bitcoin