We've found it's changed the conversation about what to build into one framed by investment and return, which has helped to take a lot of the emotion out of the discussion and decision-making process (and is also a much sounder basis for decision-making than, for example, how long things might take, or simply caving to the loudest voices[0]). So, yes, you could say it helps us manage disappointment but, whilst true, that's somewhat reductionist. What it's really enabled is an effective working relationship between our technology team and the other value-generating areas of the wider business.
In turn, the primary value that's delivered is to ensure our development teams are working on projects that (are much more likely to) add real, strategic value as we build our platform and products, as opposed to just chasing individual client requirements - requirements that would deliver short-term cash, but which would tie us in knots and lead to dead ends that would destroy our ability to scale our business.
(Of course, we don't do Shape Up in exactly the way described in the book: rather than cargo-culting, we've tailored the process to fit our business, which is a very different business to Basecamp. Happy to offer more detail if anyone is interested.)
[0] It certainly gives us a way to manage and involve those voices though.