The SOC number is a good indication of how large the pool of competition is, not necessarily how good it is at the top. It is a good all-around number when doing general research.
When you focus in on a keyword, the details may prove that the top 10 is rather hard to crack (or top3) on a key that has a low SOC. It doesn't really matter how large the overall pool is, if you can't crack the top, don't waste your time.
Two examples:
A) SOC of 2,000 may have tens of thousands of inanchor, intitle, inurl, etc. At first glance it looks competitive. However, looking at the top 10, you may be able to break in, because no one has really done good seo on the term, you can match the authority and age, etc.
B) SOC of 20 has much, much fewer inanchor, intitle, inurl. However, these sites have authority, major links, age, etc. You can see you're probably not going to rank where you want to be.
SOC is still a good general number to guide you, but it shouldn't be the end of your research as you have found. If it looks like you can't make it, you're probably right.
What I like to do is use MNF to do general research until I have some keys I'm interested in. Then plug them into Market Samurai one by one and look at the competition on the top 10. Then I know for sure if I should pursue that term or not.