No roof means less chassis rigidity, which can spoil the dynamics of the car. Engineers fix this my adding strengthening, which adds weight which again spoils the dynamics of the car. This is in theory.
In practise there is a very good chance it will make absolutely no difference to the way you experience the car on a public road. If you do intend racing the thing and lap times or 1/4 mile times are very important to you, then maybe consider a roof.
Otherwise go for it, there is just something about driving with the roof off.
I wouldn't mind one myself..
:inlove: