The problem with just turning more threads onto the front of the strut rod is that you're not really making it adjustable, you're just shortening it.
As Killer6 mentioned the point of the adjustable strut rod is so that you can make the strut rod the right length to have an LCA that moves freely without binding through its whole range of travel. The factory strut rods didn't always do that, they're kind of a "one size fits most" deal where the majority of the time they were "ok", sometimes they were actually right and occasionally they actually caused binding.
I run adjustable strut rods on all of my cars, and so far I've found that all of them have ended up being shorter than the stock non-adjustable strut rods. But I also run poly LCA bushings, and that does have an effect on the strut rod length as well. The advantage to the adjustable strut rods though is that you can set the length while they're on the car, as you run the LCA through it's range of motion to determine the right length before you get binding. I shorten mine until they bind then back off until the LCA travels freely again, so I get as much additional positive caster as I can without any binding. Remember the point of the adjustable strut rods is not to be able to add positive caster, it's to have LCA's that don't bind (although I can typically get some + caster from them). If you just cut some extra threads you'll shorten the effective length of the strut rod, but you won't have an adjustable feature. If you cut too many threads it will bind up and you'll be stuck, and if you don't cut enough you'll have to pull it off and cut more.
As for "too much positive caster", you'd have a hard time adding too much positive caster to these cars if you're running radials (in general, not by adding it with the strut rods, remember, no binding!). Remember the factory alignment specs are for bias ply's, and they're completely backward for radials. The factory specs are literally backward from what you want with radials, factory calls for negative caster and positive camber, which is AWFUL for anything other than bias ply's. With radials you want more positive caster than you can usually get with factory components, even with offset UCA bushings and adjustable strut rods a lot of cars won't get much more than +3 or +4 degrees of caster, and that's not "too much". Modern cars run +8, +9, more. The more positive caster you run the more stable the car will be at speed, and that makes for a better handling car even just tooling down the freeway. Now, for a manual steering car you might not want to get too crazy as it will add to the steering effort some. But again, unless you've got tubular or adjustable UCA's you're not going to get to a place where you've got "too much" positive caster.
