Consumer Activism's a good reason. Petty and insignificant as it may be, I became fed up with YouTube's content policies and search censorship for news topics, cut my red subscription, and put the money I had been giving toward my most watched channel's patreons. Newpipe and Skytube have been my go-to clients since.

Subscribing gives a worse experience than blocking the ads. If you're subscribed and accidentally open another youtube video, it'll pause the currently playing video with a message saying that a video is playing in another window.