You got this email because of your registration with Bowker and acquisition of ISBNs. It is harmless to do their survey, but I have found it is not worth while - they will give you a free report, but for us small or independent publishers it contains no information of value. I just delete them now.
I agree with Peter that these surveys are time consuming. I don't agree, however, that the BISG has no information of value to small publishers. They produce a report titled, "Under the Radar" that is very interesting and useful. The full report is quite expensive, but they have a free, summarized report that you can obtain on their web site.
Among the interesting tidbits in "Under the Radar" is the growth of small, independent publishers such as yourself. The numbers grew to 85,000 from 67,000 in 2002—making this one of the fastest growing segments of the publishing market.
Furthermore, "Under the Radar" details the different publishing segments. You can compare your performance against the growth of others in your category or niche.
Will the information in "Under the Radar" be earth-shatteringly useful? That depends. It will help you better understand the overall publishing market. It may also give you some ideas of niches or categories you are not currently reaching. It may not, however, radically change your tactics on how to best reach your target reader.
Thank you, Bill for your insights! I hope I haven't gone and deleted the email from them yet. LOL... or else I will have to hunt them down instead of vice versa.