Assuming the estimated result count is at all meaningful, it looks like + still does have an effect: it turns ["x"] in to meaning the same thing as [x].
So, I was skeptical that only 19k webpages would contain the actual word mars (as per the +mars query), and curious why "verbatim" search wasn't included for comparison.
So I tried verbatim search for mars, to find the estimated results count disappears. However, skipping along the pages got me the counterintuitive result that verbatim search for mars finds only 188 results - I'm pretty sure that google has indexed more mars-mentioning pages than that!
Oh, not counting the "very similar" results:
> In order to show you the most relevant results, we have omitted some entries very similar to the 188 already displayed.
Clicking this "include similar results" link gets me 45 results pages or about 450 results that mention mars found with a verbatim search.
Google has never given you all 18 million or whatever results that it says it has for a term. If you look at just mars (no quotes), it says (for me, not signed in) there are 211,000,000 results, but the results only go to page 26 with the same kind of message:
"In order to show you the most relevant results, we have omitted some entries very similar to the 260 already displayed.
If you like, you can repeat the search with the omitted results included."
even though obviously there are more results that aren't "very similar" to the 260 that have been displayed.
The assumption has always been that people don't keep clicking through tens of pages of results...they refine their search terms.
I suspect you're right in that it's never given you all the results. I'm pretty sure that it's given me many more than 300 or so results in the past, though, whereas that figure seems to be google's idea of the max any user could possibly want now before they refine their term.
I think mars is probably not the ideal term for testing this sort of thing - just too many hits. I tried mars "philip k dick" and found the same thing - 30 or so pages and then it refused to give me more results. Increasing the obscurity level somewhat, devil's vindata vanilla search vs. verbatim search did indeed show a reduced set of results for verbatim (the vanilla search results including hits on devil's vendetta)
Overall though, if you're looking for more fringe results for common search terms, millionshort.com might be worth a try, where you can remove hits from the top 10^n sites for your term it says.
It used to give you 1000 results (100 pages). Then it changed because 'nobody' goes to those last pages and it's very expensive to keep those big sets. Then it dropped to 300 or so. Then it became 'some random number between 20 and 300 depending on invisible factors'. Pretty rare to see a resultset with more than 200 pages. Even 'webcams' (of which there are 10's of millions of pages) gives only 323 results.
I seriously think that Google's end-goal is to show you only one sponsored result per search query whether it is relevant or not (there will be no real search results, it will be more like TV)
"mars" - 228m results
+mars - 19k results
+"mars" - 197m results
Assuming the estimated result count is at all meaningful, it looks like + still does have an effect: it turns ["x"] in to meaning the same thing as [x].