First the easy part of the question...when using a dry fly, there is no need for a strike indicator because you see the fly get taken.
Now the hard part. Strike indicators can be very subtle and work extremely well when used sensibly. For example, fishing a team of nymphs on a river, steering them around rocks and shallows, but like many things, strike indicators can be abused. For examle, a large SI, cast out with a couple of blobs under it, and then the rod put down and the whole lot is left to fish itself.
Many on here use a dry flie as a strike indicator and suspend a buzzer below it. Much more subtle, not likely to break any rues and very affective indeed.