I did create a PAD file and used a submission service to get the software on most download sites. I have the software listed in hundreds of download sites, and can see a good level of installations happening monthly.
Here's my opinion on the return you might get:
SEO - I agree with what's been said so far. Even if all those download sites have a link to your website, the SEO juice you get from each is going to be quite small. Anyway it's still good to have many websites linking to you (especially if they use the right keywords in the anchor text)
Awareness - Having your product listed in these sites makes it more visible to a larger group. Probably 90% of them will not be buying it, but if your doing a Grass Roots strategy, then it makes sense to try and get your SW in the hands of as many users as you can. Some may write about it (even if it's just a tweet) or tell their friends about it (and hopefully their boss!)
ROI - Most download websites let you get your software there for free! You can submit your PAD file on each site manually, or hire submission services (I use one which is cheap and quite effective). So, for a very low investment (of time and money) you can get your SW all over the web, get downloads and hopefully some deals... the ROI math is kind of easy!
Hope this helps