It's what the Packers do, and it is what most teams do, because it is what they have to do. When you have only 8 linemen active on game days, sometimes only 7, it is absolutely essential that some be prepared to play more than one position. If it isn't the backup, then a starter has to switch to accommodate the backup who can play the starter's position. Reality is what reality is. It doesn't make a lot of sense to carry 10 linemen on the game day roster just to have a different backup for every position. It isn't even common to carry 10 on the 53 man roster.