I've read different opinions on this, but I wanted to try a starter because my recent brews have not gone that well. Is it advantageous to do a starter, assuming rehydration first, with dry yeast, or is it of little benefit?
Yeast starter's goal is multiplying yeast cells so they do not have too difficult job eating through all the sugar in your wort. It is recommended to do them while using liquid yeast because they are more delicate and die much faster than dried version, thus reducing the cell count and lowering your fermentation parameters. Dry yeast does not need to be multiplied over its packet numbers in normal circumstances.
Having said that, it would be useful to make a starter with dry yeast in case of higher gravity beers. It is recommended to use more than one packet of dry yeast for gravity from about 18 Blg (~1.074 OG), but you can multiply cells from one packet in a starter to get similar result.