Originally posted by kangyio
Not really,walls will definitely become useless later but not barracks.With it veteran land unit is produced,full healing in one turn and upgrade land unit,it's always relatively important.
However, when your army gains momentum, the front line will shift very quickly. Let's say you conquer a city, which then becomes the frontier between you and the enemy civ. In the same turn, you conquer another city, further inside enemy territory. The first city you conquered in that turn is no longer the frontier, and there's no reason to station troops in there for protection. Since there won't be troops healing in the city, you don't need barracks there for healing purposes. You may want to build barracks in the second city you conquered, instead.
So, when your frontier is advancing so quickly, you wouldn't build barracks (for healing purposes) in every city, but only in certain ones along your conquest path.
As for producing land units: if you're conquering cities far away, corruption and waste will be enormous, so producing units like infantry will be out of the question (unless you're willing to rush them). If you don't build the units, it doesn't matter if they're veteran or regular.
However, upgrading land units is a good reason to justify the usefulness of barracks.