The user just wants to get something done, not think about
"correct" formats and complex UIs. Computers are good at figuring
out how to handle input of different types (up to a point,
anyway). It's a perfect match: let the user type whatever he
needs, and if it's reasonable, make the software do the right
thing with it.
This can help simplify the UI tremendously, in terms of how many
brain cells a user has to use to figure it out. It may even
remove the requirement for an
Input Hint or
Input Prompt, though
they're often seen together, as in the example above.
You might consider Structured Format
as an alternative, but that pattern works best when the input
format is entirely predictable (and usually numeric, like