I was taught not to hook in on launch... that was like the number one rule, how come so many people do it?
It seems like some guys prefer to have the kite moderately well-powered with the bar all/most of the way out, for a few reasons:
1. You don't have to work the kite as much to get going
2. You don't need as much speed to get airborne
3. When you do go for air, you can pull the bar in for extra boost
Unfortunately, if the kite is already well-powered with the bar all the way out, you can't possibly launch unhooked, even if your centerline strap is adjusted all the way. The kite would be oversheeted, giving you the drift/surge scenario previously described...
Also, some guys prefer to launch hooked in so that they have extra depower available when they are pushing the upper limit of the kite and have already done as much as they can with the centerline strap, or so that they can launch a slightly larger kite.
And then there's front-line stretch - if you don't check your lines and adjust for this, eventually the only way you can launch is hooked-in, to compensate for shorter back lines.
Last but not least, some bar systems are "leashless" - i.e.: the chickenloop is your leash (Cabrinha RECON is one of these). Launching unhooked with one of these systems means you have no leash, therefore you have to launch hooked in and pray to god that the safety can be activated fast enough and will release properly. If it's a good safety system and you launch with one hand on the trigger, this can work, but you're putting your life in the hands of a designer you've never met and a below-minimum-wage foreign manufacturing worker (or an overpaid domestic union member) - best to test this in lighter conditions first...
I prefer to tune my kites so that they can be launched unhooked - usually the manufacturer's specs are spot-on. Generally speaking, the kites will fly closer to the edge of the wind window, have less power at the edge, are somewhat more tolerant of gusts, and allow me to ride faster, and farther upwind. And I have been able to go out overpowered even though I launch unhooked - you'll get a slow drag until the kite is at about 45 degrees, then it's fine. Once the kite is in the air long enough to rule out any major problems, you can hook in and depower to a more comfortable level until you're out on the water. For jumping, I just work the kite some and build up a little extra speed - enough so that I have to push the bar all the way out to avoid getting pulled off my edge - then crank the kite hard and pull in (and hang on!!!).