As I understand, the issue is not so much getting the position data, but getting the data quickly (with low latency) to reduce mismatch between the rendered position on screen and the user's actual position. The Rift has special 250 Hz position sensors for this purpose, and even that's not fast enough. Webcams are usually only 30Hz.

http://oculusrift-blog.com/john-carmacks-message-of-latency/...