I couldn't agree more. If you're going away for a weekend all you need to take with you is your normal day rucksack plus a toothbrush and a pair of socks and pants for each day you're away. What exactly do these people with massive suitcases on wheels put in them?
If it won't fit in the luggage rack then it's not a small case! Perhaps that should be the rule; if it won't fit in the luggage rack you can't take it in the train?
This is not always as simple as it might seem. I have an 80 litre rucksack (actually 2), which technically is 10cm to tall for the regs. I am perfectly capable of slipping into a seat with that rucksack on my back, not inconvienceing anyone (yes I'm lucky I'm pretty slim), and since it's usually full of mainly clothes it can be squashed!
Now in the general case I agree with other posters about trains not being there to move house, TV sets, Wardrobes etc [some of those are entirely crazy, although I do admit students moving between home and uni and vice versa is a thing, but they tend to have less furniture than in later years

], but here are three scenarios off the top of my head where you need to take extra kit, that come to mind [I have done all three]:
1) Travelling to the Edinburgh Festivals - Even in August you never know what the weather is going to do up there! I have both had sunburn and got absolutely drenched [although not in the same year!]
2) Travelling to Corrour - Similar to Edinburgh, and you need to be prepared for the environment (waterproofs, spare boots, additional set of clothing, hiking poles etc)
3) EMF Camp or other similar four day festivals. Not only do you need clean clothes for four days, and razor/toothbrush etc, but you also need Sleeping Bag, Camping Mat, Tent etc etc [You always know you are on the right train with this one...]