[Splines]

I think his point was that it makes literal sense, but it doesn't help the process of communication

I think it does help - it implies that there is doubt (and possibly some lying involved with what's going on).

Simply stating "so and so said she went into town" is factual (although emphasizing "said" can also imply the same things that using "supposedly" does).

However, if the top-level-commenter's GF is using "supposedly" not in this way, I'd agree that'd be annoying. Maybe she's the mistrustful sort?

[noahc]

As a point of clarification she uses it in this sense:

"Supposedly, Aunt Sue is coming down this weekend"

What she means is "My mom said, "Aunt Sue is coming down this weekend"

What I interpret this to mean is "Aunt Sue says she is coming down this weekend. But, She's so flakey we shouldn't plan anything around her"

The but could really be anything, and it all depends on context, shared knowledge of the situation or person we're talking about.