I do not put on a dry fly for an indicator when I am nymphing. In my opinion, that is counter productive because a dry fly limits the depth that can be fished, AND most importantly, cannot easily be moved along the leader to adjust for water depth and speed. If fly fishers are telling you to use a dry fly as an indicator for a primary nymphing system, they are mistaken.
Perhaps the type of water that you fish is not very accommodating to a dry dropper system. I rarely use it on my small local streams. But you originally made a blanket statement about the dry dropper without any qualifying statements. Hence, I took it as an all encompassing statement.
On large western river systems that have abundant grasshopper, cicada, salmon flies, and other large terrestrials or aquatic hatches AND an abundant mayfly or caddis populations, the dry dropper is a very productive system. On these waters like the Madison, the Green, the Missouri, the Big Horn, etc, both the aquatic insects and terrestrials are present together on and in the river at the same time. During the non-hatch periods when one has to prospect for trout, using dry dropper in my experience is more productive that using a single dry fly.
One does need to cast both flies into the same current seam so drag between the flies is minimized.
On the Missouri River which has a heavy population of caddis and where the tricos can be a maddening hatch to fish, one can use the double dry system of a caddis with a following trico spinner. The caddis helps to locate the trico pattern and also to take those trout that will take the caddis. There are almost always fish that will take caddis during a trico spinner fall on the upper Missouri below Holter Dam.
Fishing Tandem Flies: Other Combinations | MidCurrent
"Seeing Double" | MidCurrent