Smaller batch in 5 gallon primary?

This might be a dumb question, but is there any reason why fermenting, say, a 2.5 gallon batch in a 5 gallon glass carboy would be bad? I'm talking about not doing a secondary, just leaving it in primary for ~1 month or so. I can see where transferring a small volume to a larger secondary could be bad, because of all the headspace and potential for oxidation, but what with the protective blanket of CO2 produced in the primary, is there any way there would be a similar problem if you weren't going to do a secondary?

No, that won't be a problem and should work just fine.

I have done many 3.5-4 gallon batches in both my 5gal carboy and 5gal buckets. Works great, and you get a better pitching rate from a vial or pack of yeast if you don't do a starter. Some brewing software is a major help scaling down, or you can take a take a 5gal recipe (or kit) and make a stronger 4gal batch. Your IBUs don't scale linearly due to decreased extraction efficiency, but the BU (bitter units):GU (gravity units) will stay close enough to the original profile to keep the beer balanced similar to the original. I brewed MoreBeer's Christmas Ale kit (AG) to the recipe, with the exception that I made it a 3.75 gal batch (aimed for 4, but boil rate was high). Came out great.