I probably seconday about 50% of the time. I find that it drops a lot more yeast out of suspension, that would otherwise drop in the keg.
Yeah, if you never move your kegs until they're drained, you don't need to secondary. But I only have one fridge for kegs, and it only holds 3, so I am rotating kegs in and out of the fridge all the time. And if you secondary before kegging, you don't have as much yeast to stir up when you move the keg.
A lot of times I secondary in a keg, then jumper cable the beer into a clean keg to get it off the flocculated yeast. That seems to work best.