Lord Hollow basically listed the pros of using 3 xx SH on block. My main gripes with the move, though, are the following:
When used in a combo in the corner, it's one of those moves that forces you out of the corner and places you in this awkward spot near the corner. So, when trying to maintain the corner, you have to give up the most damaging ender you have and end with hook charge. So if I want to end my combo with space hook near the corner, I HAVE to MB it, making me spend another bar for only a small damage increase in my combo. It's a really dumb complaint, but yeah, this is something that's always bugged me about the move.
I also hate that, when used as a poking tool via 3 xx SH, it kinda just leaves you both at neutral. The situation resets, basically, since they are most likely pushed too far for them to attempt to poke out and them being pushed away from you means you have less options to use against the opponent.
Finally, when using this as a poking tool, let's say this move actually connects. Congratulations, you got in on the opponent then poked at them with this move and sent them literally full-screen. Have fun getting back in/reloading your trait and trying to claw your way in afterwards. In this scenario, you've basically hung yourself or made life worse for you just for trying to use your spacing tools.
Because of these gripes, I've usually just skipped the middle man and cancel my 3 into Hook Charge MB (normal if the opponent doesn't have a super obvious punish option that beats it's -10 block advantage). I'm spending a bar to get in, but they can't poke out since I'm armored, I'm actually at advantage in this case, and I'm right where I want to be: right in their face. From there, I can use czar toss/b12 to continue being at advantage/tk party crasher or whatever I feel like doing, really.