Altcoins are largely useless, but they're also not very harmful to Bitcoin, so I simply prefer to ignore them.
Actually, I believe altcoins are harmful to Bitcoin in a certain way. I believe Bitcoin would have grown bigger and been more adopted than it is today if not for altcoins.
I believe this because there are so many altcoins out there that have more people lose their money. Some altcoins are created to be scams while some were created to be legit but they all have one thing in common, the creators did them to make a profit off of them. So whether it's legit or scam something happened along the line and people lost money.
Now those people who bought those coins bought with the hope that those coins would be worth something in the future, some even Believe it was a way for them to be millionaires (even if it has worked for some people) but they lost their money instead
A good number of these people that have invested and lost would assume that "cryptocurrencies are a fraud", Bitcoin included. And this hinders people who would have adopted Bitcoin.
Many bitcoiners first contact with cryptocurrency was an altcoin.
Secondly, I feel altcoins bring so much heat to the industry and Bitcoin is part of that industry. You have so many frauds and scams committed by owners of projects in the industry. These kinds of activities invite regulatory bodies and security officials to the industry, making Bitcoin a target too.
Take the instance of mixers being banned in the forum. Not all mixers are scams, but the heat of the few with illicit activities taints the image of the rest and for that reason, all have been banned from the forum.