The exploit is temporary because it's started through the browser and is not installed.
As example the 3ds:
You use browserhax as entry point and install menuhax. If you use menuhax the exploit is loaded when the 3ds trys to load your theme on booting.
For the WiiU there is nothing like menuhax public so you only have the the temporary access.
Kernel access doesn't mean total access but you can do a lot of stuff.
Load games from SD, you can install updates to you internal memory, modify things in RAM and so on.
But you don't have USB access and emunand. Iosu access is needed for the real cool stuff.
But I'm happy with the kernel access