The off-map spawn system was an artificial attempt to inject some realism into the game at the pure expense of gameplay. It was only there to make the game seem more "real". It had no gameplay benefit; in fact, it made gameplay worse.
First of all, it made building placement essentially irrelevant. That alone isn't really a big deal, but because Relic trimmed away strategic decision-making in so many other places at the same time, it meant one less meaningful decision for players to have to make. You could lose games in CoH1, and in pretty much every other RTS game in existence, thanks to poor building placement. CoH2 lacking that decision was just one less decision it forced players to make, and the whole point of a competitive game is forcing players to make decisions that have real consequences.
Second, it forced you to babysit your rally points. You could argue that this meant good players were able to use this to their advantage, and you would be right, but that doesn't make it less silly. If there was some other way besides rally points to decide which point units spawned at you could make a stronger argument that they added interesting gameplay, but the way they were implemented was just ridiculous.
And finally, they made microing and using newly created units frustrating as fuck. A good player is going to keep track of build progress and use units the moment they're created. But thanks to the off-map spawn system, units couldn't be fully controlled unit they walked onto the map proper. The most frustrating side effect of this was the fact that you couldn't issue repair orders until the unit was on the map, which means the gameplay flow of a good player who sees a repair unit is finished, selects it, clicks on a unit to repair, and then shifts his attention to other things is completely ruined. You have to keep going back to check on that unit to see if it's on the field yet or not, and the time it takes for a unit to get on the field is different for every map, every starting position, and every spawn point. A unit should be fully usable the moment it is available to be selected; anything else is a recipe for frustration. You also couldn't view and select units the moment they spawned on some maps and in some starting positions, which meant time wasted waiting for units to walk into your camera view. You would be punished for being fast and reacting immediately to new unit production.
It made for extremely frustrating and jarring moments for players, and it gave absolutely no benefit in return. It was a pure "realism" design element that actively made gameplay worse. It never should have been there in the first place.
all of this +1
also, from a perspective of maps, units coming from buildings relieves mappers from worrying about units coming into a map and how to properly implement that. most mappers arent mentally geared towards thinking about the strategic and competitive implications of things like...units coming in from off map so i need to do X Y and Z. And i need to test that feature on top of the 10000 other things that need to be considered.
Units coming from off map is an unnecessary feature that adds yet another layer to mapping that shouldent be there and everything that inverse said makes a difference in game.
FYI if Relic wanted to have units come off map for realistic campaigns or NON ladder game modes of some kind where it makes sense, im all for it. But for ladder games of any kind units should come from buildings as in COH1.
End of story.