1) To see what are the latest version of each of them and where they can be found
CGMiner always ships with the latest version of each of the kernels I mentioned earlier.
2) To see which are good for which hardware.
CGMiner also automatically scans your system and chooses the best kernel for your hardware / SDK.
3) To see which are supported by which miner.
DiabloMiner uses Diablo's kernel (duh). Phoenix use poclbm and phatk as well as diakgcn. CGMiner uses all 4. Beyond that, I'm not sure.
Again, I'm not sure I see the need for this comparison?