If you aren't getting a matching benefit or other reward for using an employer managed investment, then you shouldn't. If someone doesn't have the time, inclination, or knowledge to understand the difference then investing in an unmatched 401k is still better than not saving at all :S
Secondly, as far as "another tax shelter" there aren't any. For most people the only tax shelter available is 401(k). And the tax shelter is a very good reason to contribute to 401(k), even if there is no company match.