You view this as a person that invades right? Consider the perspective of the one being invaded.
1. Whether you are doing coop or not, if you are in Ember you can get invaded. Numerous times. Often in quick successive occasions. And now not only you have to contend with enemies, which are generally already very punishing in their own rights, you have to fight an invader too. And a new one after you win. And a new one after you win. And a new one after you win. And a new one after you win...
...
3. Correct me if I am wrong, but there is an item that will allow you to infinitely invade, right? While you don't have such luxury for coop, correct? You will always require Ember which is a finite resources that you need to manage.
There is a timer after each invasion, so you can't get invaded back to back to back. I think the only exception is the covenant areas.
The game throws ember at you like candy. I had more than a hundred of the thing at the end of my last playthrough, and that was after using them liberally throughout the game.
If you fear you might run out, just put a summon sign and help people defeat some bosses. It's easy, nets you souls and lets you practice against boss patterns.
Invasions can be a pain in the ass, especially when they happen when you're at the end of an area and out of resources, but believe me, the invaders are the one having it bad most of the time.
It's no surprise you end up with invaders rushing to kill the host without any semblance of etiquette when 70% of your invasions end up with your ass ganked by sunbros or chipped to death thanks to phantom reach. There's few things more infuriating than losing to someone spamming R1 with a straight sword or a katana and not being able to land a single parry because the servers are absolute horseshit.