There will be incentives in the future, both in terms of promoting a good seeding/leeching ratio like bittorrent ("bitswap"), as well as http://filecoin.io/
And how would you deal with materials that could get you either dead or imprisioned? Can you elect not to host content? For example, in china, you have a situation where you can be materially affected by political content.
https://www.privateinternetaccess.com/blog/2015/10/in-china-... Or what if you happen to live in a country hostile to LGBT content, like russia or Zimbabwe, where it may be dangerous to host such content?
Because you might like that company/organization/website. As the IPFS dev said in other replys in this thread, you can choose which sites you want to help
IIRC local machines by default only caches stuff for a very short time which makes sense for personal computers.
On company networks however I'd expect sysadmins to run nodes to reduce upstream bandwidth usage.
Edit: see also https://news.ycombinator.com/item?id=10329262 the specific example is the exact opposite but I see no technical reason why you cannot do this the other way around.