The system can be done better but maybe its more expensive for PCM and MK.com...
Before the presales from the site, I recall that the presales were done by Ticketmaster directly, the official site provided us with a presale code that allows us to get to ticketmaster presale and wonder what... you were able to choose your seats as ticketmaster presented you a map with sections and seats, with different colours if that seats were available when you entered or were already in someone basket or already sold, so you can you choose the better seat as possible.
The bad thing with the actual system is that if you order 5 tickets, they allocate you where they have 5 consecutive tickets, its doesnt matter if you do it before or later, so sometimes you are allocated in better or worst position depending what you ordered.
An example, if the first row have 20 seats and already 16 of them has been assigned, if you ordered 5, you will be in row 2, and the other remaing seats in first row will be assigned to someone that wants 4, or two etc etc
I think that they might puzzle with the seats and petitions so sometimes one that has ordered before another could get worst seats depending on how his order fit in the puzzle.