I read a piece not too long ago by one of the developers of WC1. He originally had it so you could select all your units at the same time and just order them to attack. The lead designer said that was too boring and easy, so he had him limit the unit selection to groups of 4.

After trying it both ways, they agreed the smaller group limit made the game more skilful and interesting to play. Ever since then RTS games have gone towards increasing the selection cap more and more! I think it’s a mistake.