With older magnetic ballasts I used to remove lamps all the time if I had a bad lamp and did not want the ballast constantly trying to start that lamp, with the hope it would actually extend the life of the ballast. Do not know if that effort was worth while or not but seemed logical at the time.
AFAIK all common electronic ballasts are designed to operate with open circuited outputs, this means you can replace a single lamp ballast with a two lamp ballast if you wish and just leave the circuit for the second lamp as open circuit or replace a three lamp ballast with a four lamp ballast.