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... I've always thought this is a bad idea ... A finger is not a mouseThere are no good or bad ideas :) Its all about time, place, knowledge of tradeoffs, execution, business, marketing, taking into account all stakeholders (user is one of them) and everything holistic :)
What you say is true, a finger is not a mouse. But we're now a decade past the release of iPhone, and by this time, the industry in general (and Apple in particular) have deep knowledge about all the tradeoffs involved here. 5 years before and this can be considered a reckless bet that requires a Steve Jobs to pull out. Now, its just natural evolution. Responsive design has been around for even longer, and changing interactions based on screen and form factors is a pretty mature problem domain now.
To be very specific:
- whether its a finger or mouse can be a runtime decision, not necessarily a compile-time or clean-slate/distribution decision for different platforms
- those decisions are already standardized enough based on existing knowledge that you can let the platform/framework (or 3p libraries) handle it out-of-the-box for you and just register multiple possibilities. instead of debating finger vs mouse - think finger and mouse
i.e. different interactions requiring entirely different apps - is not necessarily true for all of them. for a 2D application, some amount of standardization is actually good, otherwise it doesn't really help all the interaction patterns and may instead stagnate them.