This question was asked recently by someone that has seen a lot more startups than I have. The context: coffee shop business applications. In scenario, Square is absolutely essential–if you can’t take money, then you can’t serve customers (well, you can take cash, but you’ll lose a lot of business by not being able to take credit cards). Twitter, on the other hand, is mildly useful to help get more people in the door–if Twitter went away, the coffee would continue to flow.
For other types of businesses or users, the importance of these two apps is reversed. I don’t know many news journalists, but I’ve read that Twitter is absolutely crucial for keeping on top of the news and connecting with both audiences and newsmakers. Square being down wouldn’t matter at all to them (except as a story).
The point is not to praise Square or denigrate Twitter, the point is when thinking about service to sell, are you selling crucial functionality or are you selling something that is “nice to have”. If it is the former case, selling will be easier and customers will be more loyal than if it is the latter.
I’m definitely not the first to talk about these ideas, see “Is your product a vitamin or a painkiller”, but I thought the Twitter/Square/coffee shop was a nice vignette and wanted to share.