As far as I’m concerned there are two reasons for disabling speed dials: it’s faster, and your history doesn’t misbehave.
The speed is less relevant since Opera 10, although more relevant again since Opera 10.50. The history argument is far more important to me. It really throws me off that the first page I visited in a window still allows me to go back. I don’t know why someone would want to go back to the speed dial; isn’t it faster to just open a new one?
To disable the speed dial, you have you change the SpeedDialState setting in opera:config.
2: Read only, and always show speed dial
3: Disable speed dial
Set it to 3 to disable speed dial completely. If you think this will impair your ability to open pages quickly and easily, you need to take note of go to nickname (Shift + F2).