Hi Lynda,
Quite a few people do take the add-on. I think in some cases it's the thought that "it's a big wide world, who knows if I'll come this way again" (and it helps that it's a great add-on too).
To get an idea, I did take a quick look at the 2013 numbers. In terms of the add-on, the group sizes on average were about 15 (this does not include those that booked the main tour only), there were two dates around May/June where the numbers were a lot smaller and two dates at the beginning of the year when the group was much bigger. The November dates operated with 12 and 15 booked on the add-on in addition to those on the main tour only.
Kindest regards,
Julia