It's there to slow the mission down on purpose. Without the datamass, a meta mobile defense run consists of one or zero players defending the first terminal while the rest starts hunting down the next mobile defense terminal, ready to activate it the second it's possible.
Since that's not really possible without a whole lot of work (switch teleport chains or similar), people generally have no reason to not stick together and play the mission as intended.
First thing to happen after removing the datamass would be people asking why they can't hack all three terminals at once like in spy.
For clarity, I would like to be able to hack all three terminals at once, but I suspect it is not something DE wants us to do.