I think more importantly it ignores the networking effect that being in the general undergraduate program has. While I'm sure that people who attend the GS type programs at the various top universities are bright, you don't generally get to make the same connections as regular undergrads do (at Penn most GSE classes were later at night and I believe it was much harder for GSE students to take the regular undergrad classes).

I would say that while I loved everything I did while I was in school, it was the outside the classroom opportunities (student government/organizations) which were generally restricted to just undergrads that really taught me the most useful skills and connected me to a much broader network of people that I've found very helpful in my professional life. While I wouldn't say that it's a bad idea to "sneak in" I don't think it's the same experience/value as the regular undergrad program.