I see your point! Plus, it leaves open the "don´t want" possibility. And that´s more, "in case you want" is more suitable to give information or to say that some information is available (to everyone, not just to somebody in particular):
Just in case you want see a number you called or review an instant message (IM) you sent ages ago.
In case you want to know better how to move around the city click here.
In case you want to share a room, find a friend to come along with you and enjoy the surf experience together!
Anyway, nobody is going to sue you if you use that way, you would certainly be understood, just that is not the first choice of words of many people.
The "want" there has a certain ring of "if someday you need/when the need arise" or should the need arise...