Disclaimer. I ocasionally do not check luggage. I do this in two circumstances. If I have a very short trip overnight or back and forth the same day, I bring a small bag with a single change of clothes, toothbrush and shaving stuff. I put this in a backpack or duffel bag which can be squished into the overhead or under my seat (not just to be considerate but because that is what I am probably going to have to do.). The second is when I am on a medical mission and all my checked luggage is medical supplies so I have several days worth of clothes jammed into a carry on bag.
I have known this for years but it only became crystalized yesterday after hearing the young man one row back of me complain the entire flight because they had made him check his bag because there was no room in the overhead. We were sitting at the back of the plane so he would have gotten to board first had he not been goofing around and yes it pisses me off when people in the front put bags above my seat but hey, as he told everybody in ear range, he had a four hour stop-over in Vancouver plenty of time to collect his baggage.
For about the past ten years now just about every flight I have been on has taken off late because of problems stowing everybody's carry on. At the end of the flight it takes forever to deplane because of the reverse, people taking their steamer trunk out of the overhead where it has become wedged or trying to get to the back of the plane against the traffic flow to collect the steamer truck they stowed above somebody else's seat. I remember being in the Toronto airport around midnight waiting for my baggage after a flight which left an hour late because of having to stow hand baggage and thinking, "those assholes who made the flight late are already in bed".
I have obviously spent a lot of time analyzing this and I can't help but observe that most mid-large size planes actually have enough overhead space to allow every person on the flight to stow one piece of carry on luggage. If there is not enough room, it is because multiple people have stowed more than one bag especially because I only use the overhead compartment in situations like the first paragraph, this means they have ignorred all the warnings and signs about hand luggage and that the airline staff have ignorred this. This is probably because the airline staff would rather deal with one obnoxious passenger than multiple but my obnoxious friend above has a point, why is everybody else's luggage more important than his?
You can of course or some will blame the airlines for charging extra for the second (or first) bag and for not having enough baggage handlers. I have never understood why the airlines now conscious about fuel costs have not figured out that a piece of luggage weighs the same whether it is stored in the belly of the plane or in the overhead compartment. Actually I have figured it out, it is a naked cash grab. (The United Airlines flight I took back from Ecuador last year realized that the flight was full and there was going to be carry-on rage and kindly offered to check a second bag for free something I took advantage of as this is one of the two situations when I bring a carry on bag). Slow delivery of your baggage to the carousel is another issue but I expect airlines are using the excuse that everybody carries their luggage on anyway so they can get away with fewer baggage handlers.
And of course we have people who insist on carrying on way more hand baggage than they are allowed and get away with it like the obnoxious sociopath in Live Strong apparel who sat next to me on a flight from Kigali to Nairobi. He and his buddy had apparently paid (bribed) the gate agent to allow them to carry on all their expensive video equipment. He then tried to stiff the gate agent who came on the plane and wouldn't let us take off until the fee (bribe) was paid. We were of course already 2 hours late. While he was doing this he made everybody take their carry on out of the overhead compartments so he could put his stuff in and what he couldn't fit he stashed behind the seat in front of him spilling over into my leg room. Another loser on a flight from Washington to Chicago who was in business class with us was actually allowed to carry on a duffel bag almost as big as what we in Canada call a hockey bag.
Another thing that galls me is people who don't check luggage when going on vacation. Sure I can see if you are going on a short trip, that you can get away with a small amount of clothing and sundries, but I am been amazed on travelling to vacation destinations to have passengers on board who have managed to stuff 1-2 weeks of clothes into a carry on bag. Or it could be that they couldn't fit all their stuff into their 50 lb of checked luggage. Either way anti-social.
I went cycling in France with a corporate lawyer from Washington who told me he wanted as his epitaph, "Never checked luggage". He also insisted on sprinting off every morning and riding a couple of km ahead of the group, stressing out the guide, smoking cigars before dinner and only drinking the duty free scotch he had bought over from the US until one hotel in Normandy told him he was not allowed to bring his own liquor onto their terrace.