Why wouldn't the dominion use all the tools they developed? Having 5 Weyouns is better than one.
Having multiple Vorta clones seems to be incompatible with the Vorta life cycle set out by the Founders.
We know that when a particular Vorta dies and a clone is activated, the successor receives both the memories and the personality of his or her predecessor, and the new Vorta fills in seamlessly for the previous one. Multiple simultaneous clones would mean multiple sets of memories.
That being said, one could ask why there can't be several "lines" of Weyouns, where each line has its own memories and experiences. This seems, however, to be an option that the Founders are unwilling to exercise.
For instance, when the Founders are dying from Section 31's morphogenic virus, a team of Vorta doctors (all distinct) were assigned to work on a cure. When they failed, the female Changeling had them killed and had their clones activated, to bring in a "fresh perspective".
Of course, she could have activated their clones and had them work along side their counterparts — two of each mind might be better than one. At the very least, there would have been some parallel processing. But she didn't.
So it seems that the Founders are quite rigid about not activating more than one clone of any particular Vorta simultaneously, and this seems to be a custom rather than a practical issue. It could simply be a matter of ensuring that each Vorta feels an individual sense of purpose in their service to the Founders.