Boss health scales with summons, that's it. Not "all stats," like I said. Normal enemies do not scale with summons at all. I'm speaking more to Dark Souls 1 in general, as that's the team working on Bloodborne. Every DkS1 boss is infinitely easier with co-op.
I completely own up to the fact that when you said stats I was only thinking HP (even though it technically is a stat too). But there are bosses that are harder in DS1 with summons, too. Manus and 4K become massive damage sponges (especially 4K, and you want them to go down fast), and Gwyndolin gives you no advantages when you summon because he warps after a couple of hits (which your co-op buddy may or may not be there to take advantage of when he gets hit), and all of you are funneled into the same narrow hallway so you can't spread out and lure his shots or anything (actually it's worse because you don't know which person he's aiming for).
This "the game is just too hard for some people" philosophy is terrible and needs to be stamped out, it's exactly what leads people to use co-op as a safety blanket. Anyone who plays patiently and takes the time to try different strategies on enemies can become a very proficient Souls player.
Or they could quit, or cheat, or worse, never buy the game at all due to the difficulty. And that doesn't only apply to noobie gamers and folks with low skill ceilings, I know a lot of folks who are skilled gamers and I fully believe could beat a Souls game who never tried them because "they don't like super hard games" thinking that Souls games are like Boshy or something. We want people to play the game because that means more potential games. And it's not like From is backing down on the difficulty for those players with lower skill ceilings to get sales by putting in difficulty settings or something, this is the same mechanic that's been in since Demon's. Saying this a third time, they feel co-op is fine or they wouldn't put it in. And as the devs, their thoughts are all that matter ultimately.
Because the game has a reputation for difficulty, a lot of people come into it looking for anything overpowered and unbalanced by default, and they become reliant on these strategies throughout the whole game. Then they complain when invaders outskill their entire party and leave them quitting to the dashboard in rage.
Ragequitters and play to win sorts aren't necessarily co-op only one look at Xbox LIVE can you that lol) but if someone is summoning help to get them through, chances are they aren't looking for PvP and will be salty if an invader kills them no matter what the reason. Even if the invader was a nub and got a lagstab or used one of the "overpowered and unbalanced" things you mentioned to spam out a win. All invaders aren't gods of the Souls games, they have varying degrees of ability just like everyone else playing the game.
Other players take a beating early on, git gud, and become useful members of the community.
"Useful"? To whom, exactly? You're advocating that people only use co-op as a crutch, so by that logic who are those that you define as a "useful player" useful to? How are they interacting with a community and becoming "useful" if they aren't using those skills in co-op themselves and get noobies through tough areas? Because someone who solos everything isn't useful to anyone but themselves in terms of an online community. Unless of course by "useful" you mean people that become PvPers or griefers, which only helps one section of the "community" you propose as so eloquently said by another posted in this thread:
http://www.neogaf.com/forum/showpost.php?p=154953982&postcount=531