Do the numbers, Tom. Then forget it.
Raising a rocket to 30,000 meters DOES reduce the delta-V required to gain low Earth orbit, but not by very much.
On the other hand, the largest payloads ever carried by balloons to such altitudes were about four or five tonnes. The problem is displacement. To carry a payload up to 30,000 meters would require a balloon of enormous volume. (Remember, the bouyancy comes from displacement of air, and air is not very dense at such an altitude.)
If I were launching a small rocket, it might make sense. For a workhorse-type rocket with large payload, it is worthless.