Here's four reasons why I think it would be useful.
1/ It would be slow when you first did it. But from then on you could guarantee each page would load quickly. And the webmaster would have control over when they cached the pages, so it wouldn't matter. They could just do it at a slow time.
2/ With regards to sidebar data. Many sites really don't change their sidebars that much. And anyway I am a little confused. If you are saying that whenever the sidebar updates that WP Super Cache creates the page again. Then why bother using it at all on busy sites? As it would never actually cache even now? Am I missing something? Surely you would be no worse off then now. And it would be an option available to those who wanted it. Also, hard drive space really isn't such an issue these days. Even sites with thousands of pages aren't going to take up enough space when cached to be an issue.
3/ If you have a site where many visitors come in once only eg through search engine traffic. Then having all the pages ready cached can be a big deal. As otherwise say 25% of you potential audience is seeing a slow loading page. When they would have seen a fast loading page if it had been cached.
4/ In my own use of WP Super Cache I have noticed that a large portion are still being served by regular WP CAche pages, rather then WP Super Cache Pages (ie when I take a look at the 'Cache Contents' area of your admin screen). I assume that WP Super Cache pages actually load faster then their counterparts. So, if you have a 'Cache All Pages' option, then you can ensure that all the pages are cached as 'WP Super Cache' pages, rather then the 'WP Cache' pages. And hence ensure that the site runs at its optimal speed possible.
(As a side thought - some kind of option to turn pages that had been cached as WP-Cache pages into 'WP Super Cache' pages would be useful)
Another side note - I also wondered Don what you made of the .htaccess hack to your script here:
Do you think this is an improvement on the built in one in the programme? (Obviously we are all looking for speed improvements here. So I would value your thoughts)