Or, if you never need powerpoint.
Powerpoint allthough the standard, does not mean that you have to have everything compatible with it inorder to buy keynote. If you are going to be running your presentations off of a mac, where it be a laptop or desktop, then you don't need to worry about weather it will work with powerpoint or not. If you ever feel like you need to distribute your presentation to a large number of people and you are worried that they will not have keynote, then there is always the quick time export. If you are planning on using powerpoint a lot though at the office or what ever you will be doing, they I recomend that you get powerpoint, it is the standard to go by, so if you need standards then powerpoint is ofcourse the way to go.