I think text CAN be important depending on what kind of video you're making. If its, say, a lets play or a review of something, it makes sense to have text so people skimming through can see clearly what it is you're playing/reviewing etc. For example, my joke film reviews all have the name of the film I'm talking about in the thumbnail. Also if you have more than one video in a particular series, text (Even just font type and colour) helps people know that those videos go together.
As for my regular content, I don't think it matters as much. Some videos I'll use the whole title, sometimes I'll just use one keyword from the title, sometimes I won't use text at all.
Given that my videos are comedy, its really what's in the image that matters (Expressions, general goofiness etc). If I don't get across that the video's going to be funny, then people aren't going to watch.