Because it makes you go faaaaaaaast.
Sure yeah the damage output sucks, no arguement there, but the enemy have much tougher time hitting you too. Do you even know how fast this bloody thing is?
I shot a soldier from the back, quickly ran back into cover BEFORE he could even turn around to look at me, and then I emerged from cover right behind him and then shot him again. He had no idea what was happening. I was moving so fast, running up ladders, dropping from nearby ledges that I climbed faster than a double jumping scout, flanking him, running away, flanking him again that he's best bet was to simply shoot rockets at his feet and hope that I would be caught in the explosion.
I thought that "NO JUMPING" mentality would get me too, but no. Once you get the full charge going, you forget about jumping entirely. Jumping suddenly seems to be an unnecessary complication to the class. Why jump when you can go around the obstacle in mach speed for just about the same time?
I main gun spy, so the combat style of not jumping but just focusing on strafing on the ground to avoid damage and dealing as much damage as possible to the enemy is well known to me.
With this, I don't even need to jump as a scout to avoid damage. I can just circlestrafe and then change direction rapidly to avoid any kind of damage. More effective than jumping, actually. While you are jumping, the amount of time you can change direction is limited to one. In addition, while airborne, you follow a certain trajectory which experienced players will be able to predict.
Not so with strafing. You can change direction any time you want while strafing.
But the real strength of this gun comes from the fact that it's hit and run capability. I see this gun as I do gunslingers and huntsman and demoknights.
It's an entirely different way of playing the class.
as the Blaster scout, you aren't being called upon to be the assassins that scouts usually are. You are supposed to be the guerilla, the skirmisher, and reconnasiance in force. You are supposed to use your ludicrous speed to exploit the side-entrances and all the unique features of the map in order to quickly reach the enemy's back, hit it, retreat, and then come back and hit it again.
The extra speed granted by the Blaster allows this to be pulled off easily. A regular scattergun scout would die if they tried to pull something like this, although they'll probably kill someone too.
It doesn't matter that you may not kill a single opponent in the first rush into the combat. The enemy will be damaged, panicked, and looking for a health pack. They will most likely separate from the teammates to do so, and that is when you strike them again for an easy picking.
Now at this point a couple of enemy players will separate off from the group to chase after you. What're they gonna do to you? You have the superior speed, you can simply outrun anyone chasing you, including scouts. The people who chased you will be separated from the team and will be easily be picked off by you. Only class that can give you trouble is the normal scatter scout in this case, and they too can rather easily be killed thanks to your speed.
Unless they have god like aim.
Or if they have a minisentry engie on the enemy team.
But what scout ISN'T killed by a scout with a godlike aim and a minisentry engie?