Note that the "or" operator has some important downsides. For reducing bugs, using a COALESCE-like operator is IMHO more feasible. (see also: http://news.ycombinator.com/item?id=3415522)
Considering the semantics of the method (getting a user object or identifier), I don't think that's an issue. And a falsy OR also allows for using good (and falsy) null objects instead of nulls in object-oriented languages, an ability which would be lost (and lead to more reliance on NULLs, a behavior which should not be promoted) through a stricter selective operator.
The inability to trivially use null objects in conditionals is, in fact, one of the things I dislike in Ruby (where only `false` and `nil` are falsy).
