Basically, you get all the benefits maximized and then some. This is becoming more important as airlines clamp down on carry on bags. I will give you my experience and also link to a couple of articles on the same subject.
I don’t have to worry about the overhead bin. There will always be
room on my flight for my under seat bag.
I'm tall and need the room under the seat for my feet. Also being tall means reaching the bin is no problem. I also help the short folks when they need it
I will never have to check it.I don’t have to worry about gate checksI
don’t have to worry about when I board - the space under my seat is
reserved for me.
If you are going to get checked at the gate, 10kilo takes no less time at the gate than 20kilo. I've never had a problem with OH bin space. But I generally book early and checkin on line early and so far haven't ended up in the last boarding group.
I don’t have to check my bag to wander around the city. That means I
can check out of my hotel, wander, and head straight to the airport.
Haven't found a hotel yet that wont hold my bag after checkout, but 9 out of 10 times my flight home is at a crazy early time so it doesnt matter.
Public transportation is way easier with a smaller bag. Trust me on
this. I’ve done full carries and half carries. It’s way easier with a
smaller bag.It’s lighter and easier on my back.
Dont have to trust you on that, I have been in Europe 2 to 3 times a year for the last 15 years and a dozen times the 20 years prior to that. But I generally take a cab, or these days Uber, from the airport.
Even for winter travel I don’t exceed 7 kgNo worries about airline
weight limitsNo worries about budget airlines size limitsI can hide my
bag in the hotel dresser drawer. Out of sight, out of mind (less
worries about rifling through my bags - which I still lock)
No worries about weight limits for anyone, as long as you dont exceed them. There are a few airlines that have eliminated the weight limit and are strictly size based these days. Not worried about housekeeping breaking the lock on my bag to play with my skivvies
You really need to plan your wardrobe ahead of time (but once you
figure out what works it’s easier)Souvenirs need to be small (but
still possible)Customs and Immigration are suspiciousCan’t take
specialized equipment (you’ll have to rent it)It works better if you
have technical clothing, which is not cheap
For me I would have to let my luggage dictate where and what I do. For the cost of the trip, I will deal with 5kilo more weight than give up one experience.
Again, I get the carry on only concept. 75% of the time it's what I do. But I dont get the fanaticism about 7kilo. A goal maybe, but certainly not a priority. But each to their own.
Better; get PreCheck, book and checkin early, make sure that carry on really is the right size so it fits easily in the bin, take a taxi to your hotel, spend enough time at each stop to really enjoy it (and not have to mess with luggage so often), and sure, dont pack anything you arent going to want to use once you get there.