"Not necessarily, if you are doing CS at a top university and aiming to attain the highest possible grades this requires a huge investment of time and effort to most people."

I would argue that you're better off with a B and a full portfolio than an A and an empty one. It depends on where you are applying, though.

"Also it is not like a 9-5 where you can 'clock off'"

Until you have kids.

"no serious open source project is going to want your patches."

That's not necessarily true. Often either your patches are small and trivial and can be accepted or they are large and useful enough that the other developers will provide feedback. This feedback is often more useful to the student than just getting the patch accepted.

And nobody can reject your patches if it's your own project you're building.