Raymond Hettinger writes:
One thing I would not like to see happen is telling interns that their time is being well spent by pep-8 checking code in the standard library. It sends that wrong message about what constitutes an actual contribution to the core. There are plenty of useful things to do instead (we have an "easy" tag on tracker to highlight a few of them).
I have to ask for a qualification here, at least in the case of GSoC interns. Of course the intern should contribute to the code, but they are also supposed to become developers in the community. Spending a few hours checking code for PEP-8-correctness is useful training in writing good core code going forward. I agree with you that if they don't move on after a day or so, they should be told to do so. OTOH, I haven't yet met an intern who was willing and able to write in good PEP 8 style to start with, let alone one who was willing to "waste his time" doing style-checking on existing code -- is this really a problem?
I agree that they should be told that this is an investment in their skills, and at best of marginal value to Python, of course. As you point out, directing them away from core code to other projects requiring PEP 8 in their style guides is usually a good idea, too.
It's a bummer, but in mature code, almost every idea that occurs to a beginner is something that makes the code worse in some way -- that isn't always true but it happens often enough to be discouraging.
This is precisely why style-checking in the core may be a good idea for interns: assume the code is good code (it probably is), don't mess with the algorithms, but make the code "look right" according to project standards. The risk you cite is still there, but much less. It shows them what Pythonicity looks like at a deeper level than the relatively superficial guidelines in PEP 8.
Footnotes:  Not deprecatory. Consistent good looks are important.