It all depends on what you call "the average Mathcad user".
If I remember rightly, the main feature added to MP3.1 is an API. If you don't need the API, chances are you won't benefit much from MC3.1.
The rate at which (essential !) features are added to Mathcad Prime is a big joke. The features I consider essential may be added in MP5... or maybe not (no clear roadmap).
But don't worry, you are not the only one who feels bad about having purchased Mathad...