Why do I have to choose exactly 2 OR 5 roles?

As most of you already know, the new queue system allows you to choose two preferred roles -or- fill (or one preferred role and fill; pretty much the same thing). I think this actually limits player choice to a degree and I don't understand why Riot made it this way. I can only imagine that the system was made this way arbitrarily. If not, I would love it if Riot would make a blog post explaining why they are excluding 3- and 4-role preference calls.

Why do I care?
I am a player who has played with a 4-role preference for a long time. For me, my role calls haven't been "mid" or "jg or adc". It's been "anything but top", or "anything but sup", or "anything but adc". With the new queue, I can't do this. I have no choice but to choose exactly two roles (effectively cutting my role choices in half) or I have to choose "fill" and potentially play the one role I don't feel like playing. In the past, players have had no problem with me filling, minus the one role. It doesn't cause a problem, I guarantee it. I've even gotten to Plat V from Gold V in under 50 games before using this exact role call strategy.

It's truly, truly outrageous. Players can play ranked with anywhere from 1 to 5 players, but they can't choose anywhere from 2 to 5 roles in queue? That makes no sense.

However, I realize that at this point, I'm probably talking for naught. The role preference system is already designed and integrated into the GUI and it's not likely that they'll spend the money to redesign their system. Not any time soon, anyway.