There are extremes and extremes. I mean, should nobody develop software for the Mac or iPhone platforms? Maybe... but unlike Twitter, Apple is a fairly large and decently stable company which has been around for a while. Then you could look beyond that and say that developing for an Apple platform is still more of a gamble than developing for Windows, but the risk, even decently long-term, of developing for MacOS X, is fairly minimal. Twitter, on the other hand, is a free online service which is only a few years old, and isn't making money. I think where you draw the line in terms of how much risk you're willing to assume due to reliance on other businesses is up to you, but being pedantic doesn't really help.