PS: I had a old/ugly bike that I had for 16 years in Davis, CA that I still locked and it was stolen from an apt complex the one and only time I forgot to lock it. There are thieves whom ride around in cargo trucks stealing bikes wholesale, in addition to people whom just "borrow" bikes and dump them whereever.
Amsterdam is a smalll place so having a fluro green bike with a number plate of your name welded to the frame is a good deterrent.
Drilling for rivets is also dangerous when done incorrectly. You do not want to be riding at full steam when the frame cracks, it's nasty.
High carbon, stainless, and fancy stuff is where you can run into problems. In general you won't see this on car or bike frames, too expensive in bulk.
If it's a newer bike it's probably aluminum which you can usually still weld with impunity. If it was high grade alloy the heat could matter, but as far as I know nobody cares that much about bike frames. Heat treatment, or "seasoning" in the case of some aluminum is expensive.
Source: used to cut up old bikes and weld the frames into crazy chariot contraptions we would tow around
Steel is real, man. It'll bend, not break.
But in seriousness, yeah, I was following very closely behind a fellow who snapped his fork jumping over a bump in the road. I almost went over him. Despite the helmet, he lost memory of the accident. I called an ambulance for him and left my number. He called me the next day asking what had happened.
It was a nice aluminum frame, looked in good condition. Always check carefully for hairline fractures when buying a used aluminum bike.
I agree about the shitty-ness, it is less likely to attract attention and you feel less bad leaving it out on the streets but I think most people would prefer to ride a nicer bike if only people with keep their hands off your property.
Some people, particularly teenagers and students, can be quite creative in dirtying up their bike. A friend in school had glued sand to her bike, and then painted it. Quite a unique look. But it got stolen anyway, and when she found it, the thief had scraped all the sand off.
But there are many reasons why a bike might look ratty. The owner really could have had that bike for a long time, it could be second-hand (or more likely fifth-hand or something), it could have been stolen, it could have been found in the trash and refurbished, it could have been ratty'd up to prevent it from getting stolen, or it might be someone's artistic expression.
But in big cities it's certainly not unusual that the lock is the most expensive part of a bike.
You'll still have a bad looking bike, but at least it will be mechanically much nicer than your fifth-hand rusting omafiets.
Since I started doing that I never had a bike stolen in 15 years, despite living in Rotterdam and Utrecht (big cities) and buying 3 new bikes in that time.