I know what you mean... On the one hand, you need to give them a lot of demographic information so that they can make a comprehensive study, but on the other hand, you don't want to be painting a target on yourself for marketers, etc. I'd say to carefully read the privacy protection statement.... If it says something like all personal information will only be used for demographic purposes and won't be tracked beyond the scope of the survey, then it's probably safe. But if they are more vague saying that that information will only be used by the survey company and their affiliates, then I'd be a little more leery about it (after all, "affiliates" can be advertisers as well as the companies that actually request the surveys).
I've also heard it said that some surveys are really no-more than targeting advertising... I've even noticed that sometimes I'll take a survey and a week later I see the product on the shelves. It would be impossible to have the company collate all of the responses, make changes that the "survey" takers requested, distribute the products and have them out on the shelves all within a week. I'm guessing in those cases, the survey was being used more as a targeted advertisement instead of for true market research. At best, the research would be to help prepare the complaints department for issues detailed in the survey, or for later research for the "new and improved" version of the item. But not all are like that. Other times it will be a good half-year or longer before I see the product, so I know that they are really looking at these surveys to shape the actual product when it finally reaches the market.
Whether they are for true market research or for targeted advertising, I have noticed that for legit surveys (like the daily ones on FC), I have not been inundated with junk mail (either snail mail or e-mail)... Where I run into trouble are from those "surveys" that start off with a specific question right off the bat (eg Is President Obama doing a good job? Yes or no) and then ask you to choose your prize. These are not surveys, but are just marketing gimmicks. They require you to complete a bunch of other offers to claim the prize, and have general (and sometimes spamming) advertisements as well. But then with those, you're not providing a whole lot of information about yourself unless you're signing up for an offer.