There are cases where I do tests like that. There are two reasons for them.
1. It can save time debugging failures because I know whether the basic instantiation of thing was sane or not.
2. There are cases where there are complex object instantiation tests where an object is returned according to some other logic. Obviously these need to be tested.
In the first case those aren't tests for the sake of testing the API. They are sanity checks for saving time troubleshooting other test case failures.
1. It can save time debugging failures because I know whether the basic instantiation of thing was sane or not.
2. There are cases where there are complex object instantiation tests where an object is returned according to some other logic. Obviously these need to be tested.
In the first case those aren't tests for the sake of testing the API. They are sanity checks for saving time troubleshooting other test case failures.