WHY?!?!?
Sorry but after using the Nexus One with stock Android keyboard for a few months (since day 1 when it came out), I can honestly say this keyboard is WAY better.
Seriously, the biggest gripe I had with the Android keyboard was positioning the cursor in the right spot to fix/edit text. With iPhone you could do this by placing the cursor where you want but it's still hard. With Sense Keyboard you have arrow keys which I would argue are even better. It takes some getting used to yes, but like all good things, you have to be patient.