I've been having an ongoing discussion with a colleague about access to research opportunities for undergraduates. The top undergrads have their pick--REU programs at lots of different universities and labs, research for credit at Prodigal U, research for pay anywhere (easiest at Prodigal U, though). Undergrads with weak track records (ie low GPA) have a much harder time. Our department has a GPA cutoff of 3.0 to enroll in our research for credit course. Students with a lower GPA can find someone to pay them, but that can be more of a challenge when competing with much stronger students.
Thus far, I have had really great undergrads in my lab. As a TT prof, I pretty much only take top students who are enthusiastic about joining the lab, since I don't have the time or resources to waste on reluctant or completely non-productive students. I don't expect publication-quality work from an undergrad (though it sure is nice when it happens!), but I do expect them not to waste my supplies and samples doing pointless or incorrectly implemented experiments after a reasonable amount of training.
My colleague feels strongly that students who want an opportunity to do research should be allowed to do so. This colleague says we should encourage those interested in science, and doesn't want to be a gatekeeper. (I should note that this person does NOT and never has supervised the research course for credit).
I tend to agree with Prodigal Department's policy--a weaker student is unlikely to get much out of an independent research experience if they can't learn concepts in a more structured class. I know that there are some professors who would take "free" labor in the lab, regardless of prior track record, so I am sure that opening up the research course to all comers probably would not be a capacity issue. I know that as a research supervisor, I would be unlikely to give a meaty project to a weak student.
I don't think it is good to set up a student in a situation where they cannot (or are unlikely to) succeed. I don't think just anyone can do research--in order to get anything out of it, students need prior preparation. A whole summer of repeating cookbook experiments or washing glassware might help out a labor crunch in the lab, but won't do much to develop an undergrad scientist (or give them a taste of real research). This is something better left for a paid lab worker, not an undergrad research experience for credit. Having some sort of entry standard in a for-credit experience protects both the student and the professor, in my opinion.