The Rule 5 isn't a treasure trove, but that's due to changes in the rule governing it over the years.
The amateur draft, expansion and free agency have taken some of the luster off the Rule 5 over the years, as have rules regarding eligibility. In 2006, the new CBA mandated that players had to be at least four or five years away from when they originally signed. All of the above factors combined to shrink the available pool of talent.
Whether a team participates in the Rule 5 isn't so much indicative of their overall standing, but the overall stability of their roster. Contending teams may indeed have less fluid rosters and therefore, few or even zero openings on their 40-man rosters to be able participate fully or at all in the Rule 5. Their rosters are more set than those of lesser teams, and the better-heeled teams can fill needds with free agent signings. The Red Sox, for example, only had one opening on their 40-man roster before this year's Rule 5, which they filled in they filled in the second round of the Triple A phase.
For what it's worth, both Hack Wilson and Roberto Clemente were Rule 5 selections.