There are many benefits to using Bing and I would highly recommend that even small sites do so. As to why your site(s) don't rank highly in Bing, not signing up to Bing Webmaster Tools (BWT) is unlikely to have had any impact on that, but signing up will:
Allow you to submit your sitemap to Bing
Get feedback on things you could be doing better with from Bing themselves
Bing is the second most important search engine after Google
While Google is the biggest search engine out there, Bing comes second in generating traffic. The Bing market share has been on an upward trend for the last couple of years and recently hit 20% of all searches in the US. Therefore, even if the bulk of the traffic is from Google, what is coming from Bing is also crucial. You could strengthen the brand and drive conversions by expanding it.
Some of the reasons you should use both BWT and Google Webmaster Tools (GWT) are:
Allows you to submit your sitemap to both search engines (SE)
Get feedback on any issues with your site from both SE's perspective
Utilize features on one that are not available on the other, e.g. Bing Link Explorer (Bing), Google Plus Author Stats (Google), SEO Reports (Bing)
The Bing webmaster tools are very helpful
When you navigate to the Diagnostics and Tools section of Bing Webmaster Tools, the SEO analyzer stands out. In addition to outlining the issues on your website from SEO point of view, the SEO analyzer includes recommendations for actions. Once all the issues are addressed, a report on every page can be generated to view the new results. This will help you check the severity of any issue and comprehensive explanation.
To help with your Bing/Yahoo rankings, I would suggest the following:
Sign up to BWT
Compare your rankings for keywords on Bing and Google
Analyze the competition for your keywords on Bing and Google
If you are experiencing large discrepencies between the two search engines, then see what the sites/pages ahead of you are doing better than you