IMO they are a must, even for a car that only gets street miles.
Think of it this way; A uni-body car is like a shoe box with a lid. The lid keeps things from moving too far, but things can still move around a little bit within the confines of the lid as it twists and lifts. If you tape the lid down, similar to adding subframe connectors, the twist and distortion becomes greatly diminished and rigidity is greatly improved. Rigidity is necessary to allow the suspension to properly function at suspending the car. If the car is flexing, you turn the whole body into a variable of the suspension that you cannot control.
If you want to see the results they provide, before you put them in put a floor jack under one of the front control arms, and crank up that corner. Then open and try to close the door. It ain't easy if it even closes at all. After the sub frame connectors are in, try it again and observe the difference.
In terms of strength, even the bolt in, tube style that don't attach to the board will yield dramatic improvements, especially if you weld the ends. The next step up are the contoured weld in units, then through the floor weld in.
For a street car you will immediately notice a reduction in the squeaks and rattles of normal driving. Turn in and cornering becomes more stable, and if you ever do launch away from a stop light, it is straighter and more predictable.
DO IT!!