Psion said:
Quite simple, really.
Minis are representations of creatures. And creatures are not square. Once minis fail to represent the creatures they supposedly represent, you have defeated one of the primary purposes of having them.
I am reminded of the "spherical cow" joke:
http://wildlink.com/humour/spherical_cow.htm
I don't think you're right on this one - in 3.5 minis are not representations of creatures, they are representations of the amount of area that creature takes up when that creature spins in a circle.
Basically, the problem is that long and thin creatures are inherently incompatible with a combat system that does not allow for facing. Square bases go a long way toward fixing that problem.