I think the frustration might stem from the way (most) plugins work in the rest of the WordPress world: they add functionality that is accessed via a widget in an aside / sidebar, via a shortcode, or, if the user chooses, via a template tag directly in the theme.
Since the UI of SP lives entirely in a single WP “page,” plugins mostly can’t be exploited in widgets, because widgets live in sidebars and you just can’t have a sidebar in a page in WP. Similarly, you can’t use shortcode because shortcodes live in the content of a page, and SP creates all of its content dynamically within the page.
SP is inherently flexible, and that’s a very good thing. Unfortunately, it’s not practical for most end-users to take advantage of that flexibility unless they also have experience as WP theme developers. To be most successful, I’m guessing SP wants to be attractive to the average end-user while still having great bells and whistles for the theme developer.
Mr Papa mentioned that he’d rather spend time in the support forum helping folks than build a less flexible SP. That’s commendable, but here’s a suggestion that would be more efficient for all involved:
Complete the SP documentation, and make sure it’s presented in a linear way and written to be as understandable as possible for the average end user (not the average WP theme developer.) Also, don’t release a plugin until its documentation is complete and written to the same standard as the SP core docs.
Since access to support is a big part of the SP business model, put all the documentation (except for installation and upgrade) behind the paywall.
The advantage to users is obvious. The advantage to the SP staff is considerable, too: complete, clear documentation means staff spends less time answering questions in the forums — time that can be spent working on the product itself, which I’m sure is a big priority.