Aaranged - 10:20 pm on Mar 10, 2012 (gmt 0)
Just from a logic perspective, does it make sense to hide the most detailed, actionable, (relatively) content-rich pages from the search engines? At at very basic level I'd say no.
And this strikes me as a classic bad tradeoff. That is, the price of improving the ranking of your category pages shouldn't be entirely borking your product detail pages. Put another way, chasing short tail keyword traffic shouldn't be done at the expense of your long tail traffic.
Furthermore, you may actually damage the rankings and keyword reach of those category pages for a number of reasons. If you have (sensible) breadcrumbs product pages going to be pushing and concentrating link juice upwards to their parents. If there's social interactions on your product pages they're lost to view. If there's spiderable UGC (like reviews)- often great, original, keyword-rich fodder - that's going to be invisible to the search engines. And, of course, fundamentally you're going to be losing the semantic strength of your product titles, descriptions and (perhaps) images and videos.
Some of this might be addressed with combining (no)index and (no)follow directives, but heavens knows only an SEO could up with something so convoluted for such basic pages (even with more complex day-to-day ecommerce situations like paginated results and URLs spawned by faceted browsing I generally think you're better off going big or going home - nix the pages altogether, or expose them and their linkages fully).
So, and interesting proposition but, yes, I think it is a crazy thought. :)