The kroot are mundane at best (someone please prove me wrong), and i decided that if and when i start a tau army, it won't have any kroot at all. The very fact that you have to combine all the kroot into one unit and keep conherency takes away the versatility of the the different squads.
You can also look at that as adding cheap-as-dirt 7 point flesh shields to your Krootox, who would otherwise be picked out individually.
The truth is, as far as I can tell, Kroot are there to counterassault if the enemy gets too close. That's it. All they're good for. However, at that particular role, they're awesome. Follow me here for a minute...
In a counter-assault troop, all you care about is keeping the assault OFF of your Firewarriors. Beyond that, you want the counter-assault to be cheaper than the firewarriors (as cheap as possible really). If they can actually kill anything in HtH then all the better, but it's not neccisary at all. Also, it's an added bonus if they can soak up fire and/or kill things occasionally with guns.
The Kroot meet all the above criteria. They're Cheap-As-Dirt™. It's MUCH better to loose a 7-point Kroot than a 10-point Firewarrior. Advance the kroot up just in front of your Firewarriors and they have to be dealt with before an assault can reach the real meat-and-potatoes of your army. Shooting attacks also must engage the Kroot screen, meaning you're loosing fewer points every time one dies than if a Firewarrior had been hit.
And when the Assault comes, you only need one of them alive. That's enough to force a sweeping advance where you get yet another turn of free shooting before your Firewarriors are actually engaged.
If you actually have more alive when the Assault reaches you, then goodie. The Kroot, while lackluster in performance, are still anough to get a couple of casualties (which is a couple more that your Firewarriors don't have to kill on the sweep). And hey, occasionally things die to their shooting as well.
Trust me, if I can find a use for an Eldar Guardian, I can find a use for anything
But still that works best in a static fireline/layered defense, and I've personally spent a little too much time playing Eldar to ever sit completly still.
Update: Oh yeah, and more info on the Kroot (although not relating to the Tau army, still a great read) is at http://www.eldaronline.com/community/index.php?board=4;action=display;threadid=11303