Software design patterns as a goal are perfectly appropriate for APIs - the point is that if you use a well known pattern then programmers working against the API will be familiar with what's going on - if they're familiar with the pattern. If they're not familiar it gives them an opportunity to learn about it, rather than some messed up emergent rats-nest of functions just pulled out of the air.

Even if the functions you provide only loosely fit an established pattern it's still worth "adapting" with verbose notes about how it doesn't conform to said pattern.

Actually I would go so far as to say there is value in developing within a pattern even if you're not building an API, if you expect that a number of third parties may have to maintain or develop your code for years to come.

However, if you're working against problem domain that is well understood in your organisation (technically specialised, low level) it doesn't make sense to force a pattern upon it. It is reasonable to expect that third parties should know what's going on already.