What WTF provides is the ability to come up with an idea and get a quick gauge as to whether it has merit (it works with projects at various stages from new ideas to coming up with new features for existing projects and anything in-between).
For example, you have likely heard of "Facebook killer" apps and "New way to organize your inbox"-type ideas in the past and the general reaction is "Oh ffs, not another <insert product here> clone" but most ideas still have some merit even if their initial briefs are rather ambitious and somewhat tenuous.
Your Facebook killer may be too ambitious but with a bit of focus you may be able to come up with a smaller, more niche product that benefits a particular demographic given feedback and some direction.
This is what WTF provides: Sure, it will not give you a definite "If you build it, they will come" answer but it might just be able to help you salvage something that doesn't look like it is going anywhere.
In addition to helping you shape a project, it may also prevent you from spending time and money on something that just will not work as a result of feedback so you will know fast if it has any potential before spending time and money on it.
AS to stealing your idea, this is rarely the case. The thing that sinks most projects is lack of focus and aiming in the wrong direction - something WTF is designed to help prevent.
Thanks very much for your insight... this is exactly what WTF was designed for.
Mick