Hello revile, by earplugs, do you mean those that cut out sound? Or are you referring to the studs that stick out of a musician's/vocalist's ear(s)? Those are in-ear monitors.
I can think of many reasons to NOT use earplugs. Instead, try getting the volume/pan of individual instruments to be optimal, with the vocals cutting through all of them. Local studios are normally so small that the drums are already very loud without amplification. If your vocalist cannot hear him/herself because of that, you can choose to play softer, play in another studio, or get monitor headphones for the vocalist if the studio has a main mixer.