Because it's lazier -- less planning involved. If you can upgrade units easily, you don't ever have to worry about a sneak attack. As soon as war is declared, it's much easier to get your force ready.
And you'd have to MOD the AI to take advantage of the new upgrade system as well.
But hey, some people want to play an easy game -- they're allowed. I just think they're missing out by making one strategy easier / better than others. With upgrade costs as they are, you actually have to think -- should I upgrade this unit, or should I just build a new one?