There is no reason to have a 5 gallon carboy that I can see, unless you are making wine in 5 gallon batches.
To use as a secondary for a 5 gallon batch of beer. Many brewers like me still do a secondary...depends on the style and clarity I can get in a long primary.
Nothing wrong with racking to 6 gal carboy either, as most beer secondary isn't long enough for oxidation to matter in the added headspace.
that said, 5 gal carboys are LOTS cheaper at my LHBS than 6 or 6.5 gal demi-johns. I primary in buckets, secondary in glass carboys, which let me see how the clearing process is going.