This is really a long and tiresome debate… I personally have argued this point for a long time to no avail with Zen Cart forum posters… Even linked to Sebastian for verification. But, alas, the debate drags on.
I have said over and over the major search engines, Google included, DO NOT use the keyword tag…. Like talking to the wall. So, to finally put this subject to rest and answer this with absoluteness… Matt Cutts has clearly stated in no uncertain terms that Google Does Not Use the Meta Keyword Tag to Rank Websites!
Blocking my page in robots.txt will prevent indexing.. This is just not true, your robots.txt is a tool used to prevent crawling… not indexing. While blocking a page in robots.txt will prevent well behaved spiders from crawling these pages, it will not prevent indexing, remove a page from the index, nor block the flow of PageRank to the page.
Choosing the right keywords take time, deep thought and effort. When choosing keywords to expand your search engine visibility you should be asking yourself some of these questions.