I understand the sentiment, but those are the worst reasons ever...
Reason 1: Because you CAN. It's also called FREEDOM of CHOICE. (A concept that seems to be completely alien to most Apple users.)
Freedom of choice is not a foreign ideas here. JB is already a very common topic on this forum.
Reason 2: To use OPEN SOURCE software and/or to write software for the device in a language like Java - something that Apple neither condones nor supports nor allows.
The Open Source issue is do to restrictions in the GPL, not Apple's ToS or EULA. Besides, high profile Open Source apps are available in the store right now. As for Java; the restrictions are with using common shared installs of those frameworks. If you want to use those techs in your app, just include the binaries.
Reason 3: Just for the kicks of it and to show the folks in Cupertino the middle finger. You know, like installing Mac OS X on a PC, only this time we break Apple's tiny Walled Garden and put the competitor's software on their precious little digital prison.
If someone bought an iPad to do this, they're not really showing Apple the 'middle finger'. BTW, installing OS X on a PC is not a valid use of the license. This is not a good comparison to installing an Open Source OS on a device.
For the record, I get the 'just for kicks' part and I do have iDroid installed on one of my devices.
The reason people don't care much about it is, while it's entertaining to play with, it's a pretty poor experience on the iDevices. It's missing the hardware buttons, the drivers are very buggy and since this is not a 'blessed' android device, no proprietary apps like Google Maps or Google Market. I'm sure the performance issues could be addressed in time, but I doubt before the release of android 3.0. At which point they need to start over. Someday iDroid will be useful, but it's going to be a long time.