My understanding is in WoW you are often required to use them to be competitive and that later many groups/guilds will insist you use them.
Only because in raids, there are spell effects that have to be decursed within two cooldowns or it'll wipe the entire raid. Blizz is designing content with the assumption that people are using Decursive and other tools to deal with dispellables.
Some addons even tell you what is going on in Boss fights and what attacks the Boss it going to use next.
This is now native to the UI. There were always spell emotes to tell you when critical stuff was happening.
I dislike the addon(s) which show you exactly how much cast time is left on a spell as you’re casting it.
That's not an addon, cast bars were always in the native UI, even in the beta.
that they tell you when anyone in the party gets agro (you used to have to watch the screen).
Watching the screen wouldn't tell you anything about aggro except where the boss was facing; it certainly wouldn't tell you you were about to pull of the tank until you did. Not knowing your aggro means pulling back on your DPS a lot more than you have to, and that matters nowadays in a fight like Patchwerk, which is a DPS race/gear check where not doing the most DPS you can will wipe the entire raid. Blizz is designing encounters with some notion that you'll be able to monitor your threat.
I see why people like them, just think they make games a bit too easymode.
Your notion of what isn't "easy mode" seems absurd. You envision a game where curse effects that you have no way of detecting are wiping raids, where you have no idea what kind of threat you're pulling relative to the tank until the boss turns and cleaves the DPS, where you have no idea how to complete quests, where it's impossible to time your dots and rotation in a DPS race because you have no idea how long your spells take to cast under various circumstances.
The game's plenty hard as it is, without demanding that players play it half-blind and one-handed.
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