Originally Posted by BestXXXPorn

PornTube would have to trademark the name MobilePornTube in order to demand the domain. You can't tack on additional words and have the trademark hold. That being said, you could trademark it first if you really wanted to protect yourself.

But as everyone else said, it's all about what you plan to do with it. If you believe the site you plan on building is worth a $3k investment on a domain name then go for it!