It's quite practical if you understand its proper function. That does not mean jumping guard when someone is trying to punch you in the face.
As a general rule, if a fight goes to the ground you want to be on top. This gives you the advantage for striking and grappling as well as superior mobility in case you need to bail out quickly.
Unfortunately, you can't always get what you want. If someone is attacking you, there's a good chance that they may be bigger and more agressive than you are and they may end up on top. Imagine a 230-pound former high-school football player blindside tackling you in a parking lot. You really, really don't want to have the big guy sitting on your chest pounding your brains out. This is where the guard comes in. It's an equalizer for when someone gets on top of you, either because they're bigger or because they're a better wrestler or because they just got the drop on you. If you can establish the guard when someone takes you down, then you have a chance to neutralize their attacks, get your bearings, and then go on the attack. In a street self-defense scenario, I would place a higher priority on sweeping or escaping then on using submissions from the guard. (Up-kicks can also be very effective if you are using an open guard.)
