What's this obsession with travelling ultra-light?

As you said, travelling light is about ease of movement, not having to lug around 20kgs, not having to worry about your backpack stored under the bus when you pull up somewhere in the middle of the night (because your luggage is at your feet in the bus), not having to be frustrated when the airline misplaces your bag and you have to make do without your stuff.
That said, I'm actually like you, I don't generally travel light, I can't be bothered having to wash clothes every other day, I want to be able to change into clean, dry clothes at the end of a day spent sweating and getting caught in the rain or covered in mud. I like to be able to use my own mask and snorkel rather than renting one which fogs up and was last used by who knows who and whether it was properly washed. I like not finding myself somewhere and running out of shampoo or being able to buy that souvenir because I have space to store it.
It's all about finding the balance that's right for you.

I travel light,though i used to travel heavy ;-)

Cheaper (no paying for baggage charges),easier to move around,faster (no waiting for luggage to arrive in the airport etc.),safer (everything is on or near me).

The negatives for me are far outweighed by the positives,but of course there are some....

I have to do a little washing every day (or max. every second day).
I rent snorkelling stuff etc.Unless it is purely a beach holiday,in which case I include that.,,usually my snorkelling might be 3 or 4 days spread over 3 months...that is not worth carrying the gear for me.

I rarely buy souvenirs until my last stop.If I do buy them before,I buy small and light ones!

I do usually carry a book,and I swap it when I've finished it.

I don't care at all about 'dressing up nice''...I've been into bars and restaurants all over the world with my 'travel gear',and no-one has ever refused me entry.Of coures I'm not eating in 5 star places ;-)

BTW I wouldn't consider my bag 'tiny'.....I use a 40l,and carry around 6-7kg.

That is enough for all I need,and that can be carried on as 'hand luggage' on planes,buses etc.

I can definitely see the wisdom and benefits of travelling light (winter could be more challenging). Its all about mobility, cost, and ensuring your stuff is with you at all times. I travel on the mid size range. Enough for comfort and not having to do laundry every other day but not light enough to store my bags with me. I carry a 85L bag, which technically is only filled to about 50L-60L, but i do this so I can buy gifts at the end of my trip. So far I've only travelled to more developed places, but I'm planning on doing a longer trip to South America, which I will pack on the lighter side.

There are pros and cons to either method. But overall travelling on the light side is probably better as an on the go budget traveller.

I started travelling with a large backpack, but since a few years I switched to a small 22 litre backpack. To be honest I did only short trips in these years usually not longer than a week, the last one was 10 days.

I don't like shirts I wear trousers(one on me, the other one in the bag) and shirt and T-shirts I wear a proper shoe(a light leather hiking shoe that is comfortable to wear at all times. I also pack a pair of very thin flip-flops for the bathroom.

I don't know what do you mean by nice, but no I don't go to the opera or 3 star restuarants. I just continue my usual lifestyle when I'm travelling. I meet all kind of people, backpackers too, but I can't remember any grubby ones.

I don't do any sports(other than hiking) so I don't need any gear. When travlling I do only short day hikes, where I dont need any special gear. If I go for dedicaed hiking trip with tent, sleeping bag, etc. that's diferent of course.

I always carry books and maps. I don't take souvenirs only small/light/flat ones: maps, brochures, a magazine, coins, tickets. It would be nice sometimes to buy books and food, sometimes I have some spare space for it and I have also a textile bag which I can fill on the way home when I'm travelling by train. Last time returning from Italy by plane it was a really pity I couldn't take home of bottle of wine and olive oil because of the silly regulations, but I fugure it's not worth paying for any extra checked-in baggage for as much extra fee as my whole ticket for the sake of a bottle of wine.

There's only one thing I hated: that is washing and finding an opportunity for it, but this was a problem only on this last 10 day trip. I can take enough clothes to last a week, but that wasn't too different when I had a large backpack on the longer trips.

After this recent trip I was thinking back and realised having a large backpack wouldn't made too much difference(only the extra fee on the plane and a few euros here and there for lockers) on this particular trip, but the thing is I had an excellent trip and I didn't really needed more stuff for this trip so why bother?

winter could be more challenging

But in winter you can always keep your warm clothes on you, while in other seasons when it's not cool enough(that's on reason I prefer autumn and spring over summer) you need put your jacket and sweater in your bag(and they're relatively bulky even when compressed to the smallest size) or carry it in your hand or on your waist.

mouaka, South America is one of the hardest places to pack for as you go from hot, humid jungle to hot dry desert to cold high-mountains, to glaciers, to cities, to all kinds of places and climates and situations. You basically need to pack for every eventuality and it's a real pain.

I suppose I can always buy and sell what i need and don't need. Some seasoned travellers have done that. Plus I'm planning on city trekking with weekend excursions, so hopefully that will reduce the need for heavier loads.