For the MGS series though there's a lot missed out by not playing yourself, for example if you don't know a thing about psycho mantis, his ability to tell you your save games or to make your controller move across the ground (for those who didn't understand how the vibration in the controller worked) it was pretty awesome. Additionally MGS 3 for example, if you play the entire game without killing anyone (for example using the tranq gun), there's a boss fight where you're supposed to encounter the ghosts of the people you killed but won't if you hadn't killed anyone. The MGS series specifically is just chock full of little things you'd miss out on if you just watch a long play.