This is a counterintuitive answer, but "no" - it's not a bad idea.
What I think you are really asking is "Is it smart to include links to sites outside of my hubpages subdomain? Won't visitors leave my pages and go elsewhere?"
The answer is yes, they will, but that is ok.
I found that my most successful and authoritative hubs include a lot of links to outside resources. You want Google to view your hub as authoritative, but if you only link to Amazon, it tells Google a story - that you are really interested in the income, and not providing authoritative information.
Look at Patty Inglish's work. She always has a lot of outbound links, but is a very good hubber.