I understand the uses of adjustable lowers, but for a mildly modified street car, wouldn't it be possible to get all the required adjustments needed by just using adjustable uppers instead of adjustable uppers and lowers?

It depends on what you want to adjust...

Personally I would think lowers if I want to adjust tracking and uppers if I want to adjust pinion angle. It would be very easy to mess up tracking while adjusting pinion angle with lowers.

So if you trust your tracking angle, you do not need adjustable lowers.

Part of it is the way the adjustable lowers are built... If you're building an arm with rod-ends on both ends which thread on, it takes very little extra effort to put left-handed threads on one end instead of right-handed on both ends, and wha-la, you've got an adjustable arm.

Providing adjustable corrections to tracking errors makes sense. Instead of compensating for a thrust angle which is not dead center by making adjustments to the front suspension, with adjustable rear lowers you could correct for tracking discrepancies inherent to the frame to put the thrust angle back down the center of the car leaving the front suspension to worry about it's own issues.

Part of it is the way the adjustable lowers are built... If you're building an arm with rod-ends on both ends which thread on, it takes very little extra effort to put left-handed threads on one end instead of right-handed on both ends, and wha-la, you've got an adjustable arm. Actually they are adjustable simply by virtue of having threaded in ends. The only difference with reverse threaded ends is not having to remove the "arm" for adjustment. Of course the reverse threads also allow the arm to self-adjust...

So yet another design tradeoff: safety factor versus adjustment ease.

Actually they are adjustable simply by virtue of having threaded in ends. The only difference with reverse threaded ends is not having to remove the "arm" for adjustment. Of course the reverse threads also allow the arm to self-adjust...

So yet another design tradeoff: safety factor versus adjustment ease.True... but I'm not worried about the safety. I'll just have to check the jam-nuts once in a while when I'm under the car, but I'm not concerned at all about them coming loose.

Bottom line seems to be, though, if you don't have a measureable amount of tracking discrepancy the adjustable lowers seem to be overkill. Though they would provide greater flexibility for future more in depth modifications I suppose.

I used adjustable uppers and lowers to help center the wheel in the wheelwell to gain some additional tire clearance. I had a chassis shop check for squareness and tracking after I finished. A little adjustment goes a long way...