Assuming your keyboard layout isn't too different from the norm, the pause key is small and difficult to reach with the left hand. More modern Paradox games use the spacebar for pausing because it allows the player to keep their hand on the pause button without having their arm in an uncomfortable position while playing. As you said, you can just use the hourglass to pause, but that's less efficient. I realize that EU2 isn't exactly a very fast-paced game, but if you pause during almost every single decision like I do, it gets annoying fast to use the pause key or hourglass icon to do it.
When I say every single decision I refer to just about anything in the game requiring user action, be it training soldiers, giving orders to explorers or placing merchants. I dislike letting game time go to waste, so I use frequent pauses as a means to micromanage my empire. It also helps a lot in war when trying to get the right timing to attack or retreat. You can use autopause for some things, but not everything.
press = {
id = "BUTTON_PAUSE_ALTERNATE"
bitmap = "empty.bmp"
shortcut = " " # replace shortcut = F9.
background = "btn_bar_pause.bmp"
lower = "BUTTON_0050"
upper = ""
}