DBM is necessary for proper raiding. Even if you think that you don't need it, other people need that you use it, because it adds /say clouds above your head when important mechanics occur. And default UI is not well thought when it comes to bosses. Boss will use some abilities on cooldown (e.g. each 30 seconds), but there's nothing in default UI to display that, yet I should be prepared for that ability.
Encounter designers used to add extra bars for important abilities in Legion, so you could track them via Boss HP/Energy/Extra bars, but it seems that they decided to stop doing that. I don't like any addons and prefer to play as clean as possible, so I know what I'm talking about. I'm trying to ditch boss mods every new patch, I ditched weak auras, but boss mods: only in heroic where performance does not matter that much.