While I don't take oversized luggage, or check anything, I'd be tempted to carryon a -slightly- larger bag if/because:
If I was flying on short notice to a destination for more than a week AND I expected to be much too busy to do laundry, and/or I expected to have to attend multiple dress parties. Normally I dress sorta casually and have no problems sending clothes to be cleaned while away. I usually pack for 6 days in a single legal carryon and a smaller softsided briefcase.
Because the TSA might damage/steal/soil my belongings during the luggage search. They have no accountability and seem to fost any responsibility for claims onto the airlines who in turn point a finger back to the TSA.
Because I avoid having to visit the pre-security check-in desk. This is one less bothersome line to wait in.
Because I avoid having my luggage sent to Atlanta because the airlines run too tight of a schedule to realistically make all my connecting flights. While I can pull an OJ and sprint through the concourse, my checked luggage will be taking the slow, circuitous route....to Georgia. Even though I label my luggage VERY WELL with all sorts of contact info painted on the cases (name, address, phones, email) and taped to the inside or on an engraved pet tag attached to the handle on all three lost bags I've had, I've never been contacted.
Because if I decide I need a sweater RIGHT NOW, I have access to it.
Because the great majority of my flights are domestic. Waiting for a checked bag often adds 25-100% to the time I have to be at the airport.
This basicly boils down to two items.
I don't trust others AND don't want to involve others when it comes to my property.
I don't want to waste time.
So there! ;-)
Next time you see me carryon a garment bag, go ahead and sneer. It might just make up for all the pitfalls I've avoided - naaaaa!