Once you start getting into the more involved group projects, you'll need some decent screen real estate.
11" isn't much when you have Google docs opened, as well as one article in a different window and you're using them both. 13" is way better.
Last august i decided it was time to sell my 13"MBP. spent a day with a 11" MBA a 13" and 15" MBP. 11" ended up being way too small for multi-tasking. and my tablet was already 10. so i might as well have added a keyboard to my tablet.
so it ended up being between the 13" and the 15". so trust me, for huge group project, you need at least a 13"
i have spent 10 years as a bench scientist and now i am in law school. the 11" MBA has been perfect for both. science is highly collaborative, so i can easily use it to show data in small group settings. for larger meetings everyone plugs their computer into a projector so that's moot.
for working at my desk i have a cheap external monitor for modeling, large spreadsheets etc.
i think this is going to be a matter of personal opinion. obviously times have changed since i was an undergrad - most kids didn't carry around laptops until the end of school and those were nowhere near as light and portable as today's machines. however - portability for a college kid is just as important as for a professional, if not more so.
i think it's essential that everyone try before you buy as it's definitely going to be a matter of personal taste. your argument for the 13" is perfectly reasonable but for me the 11" is a better fit.
i would also like to point out that i walked into the apple store set on the 13" mba with no interest in the 11 - for many of the reasons you alluded to your in your post! it wasn't until after i played with it that i realized the 11" was the better choice for me.
also - i agree entirely that the choice is between the 11" and the 15. at least for me - did i want ultralight or ultra portable... so at the price difference between the 11 mba and the 15" rMBP - i went with the mba