One of the nice parts of using a Kindle is that it syncs your page between all of your devices, so if I want to quickly open the app on my phone and read a few pages I'm right where I was when I was last on my Kindle. When I get back to my Kindle, it has the new location synced.
I read a lot of non-fiction. One nice feature that it has on WiFi is the ability to highlight a word or phrase and it will display a definition or a Wikipedia article/snippet right there on the device. Its an easy and fast way to get more information out of your readings, especially when I'm at the pub with only my Kindle.
Another strong reason why you probably need WiFi on a Kindle at least some of the time is because WiFi is the primary way it gets content. You can get a USB cable to load stuff on it, but chances are if you've bought a Kindle you bought it to use the Kindle store.
Also, I'm pretty sure it downloads a bunch of ads at once when it syncs, and then shows them over time. I've had it disconnected from WiFi for a few weeks before, and it still showed ads. So even if you only connected it for a few minutes to sync everything and then you disconnected until you wanted a new book from the Kindle store, you would still see ads.