We will get two so that we can have a macro and portrait set up with two flashes (operating wirelessly from the camera master control). We are considering getting two Canon 430 EX II, but could make one of them the 600 EX-RT if there was a compelling reason to spend h extra money.
The 430ex2 ("small" flash line) cannot act as a master, so you have to use the 7d pop-up flash as master - the downside is that you cannot use remote hss in this case which might be very important for portraits if using small depth of field and thus shutter speeds faster than 1/250s.
You probably already know about the downside of the optical remote link - it requires line of sight between the flashes or a very confined space where the walls reflect the optical remote signal. On very bright days the optical signal can also be problematic, and the distance cannot be too large. Ymmv here, but if you run into problems you can either buy 3rd party radio triggers or exchange the 430ex flashes for 600rt ones, though the latter at a considerably higher price.
