It's around the same price, they cost like $35 from standard retailers (less on resale markets) so its around the cost of an Arduino anyways which you'd have to mess with every time you wanted to use it for anything else, and by having a dedicated one, he then guarantees he has one around because he can't really use it for anything else (like inside another project) :-)
Typical misunderstanding. The problem is that I also had some difficulties programmig an ATtiny85 with an Arduino Ono as ISP with arduino code. Maybe therefore he used the standard tools like AVR Studio and AVRISP MkII.
