I think a Custom Rom does not mean to only to Port Android to Windows phone. Porting Android to Windows Phone would be cool but a Custom Rom gives you plenty of other options is not jut porting Android. (Correct me If I am wrong)
Also some of us we cannot just buy any phone we want any time. Sometimes you get what you can get. Phones are expensive. Still if you have the opportunity to install a custom ROM or improve the options the phone gives you, why not? In my case I got a Titan via Windows Challenge, it was the only option they had available. It is not an option for me to spend now hundreds of dollars on a lets say a new Sensation XL. I have the feeling that there is a few people in my situation.