I've always thought the same about the uneconomical part. Not to mention you're throwing more attitudes into the band and vocalists can get egotistical
I honestly prefer when neither vocalist is restricted to an instrument.
I've always thought that about keyboardists/mackbookers though. Takes up too much pre-determined space, where as a vocalist is welcome to go all over, including barriers/the crowd.
But if the band calls for two vocalists and a keyboardist then more power to them.