The point about timers was twofold: that you get the timer if anyone on the team had vision of Dragon (or other monsters) when they died, and that it automatically does something that was a skill-differentiating-factor before.
E.g. if you have a ward at Dragon which sees Dragon die, but no one on the team actually noticed the enemy taking it, you still get the timer. Therefore the timer compensates for your lack of map awareness.
E.g. You take Dragon and a teamfight immediately breaks out before you get a chance to note the time. The timer covers you and you can check later and be like "oh it's back in 90 seconds better go set up vision".
Timing monster spawns was an important part of jungling and jungler skill levels, and adding timers reduced the skill-cap if you will.
On the plus side, you still do have to actually press tab to see the timer. So when I'm mid and bot lane takes a dragon, and I notice 5 minutes later that it's spawning in a minute and nobody has started to set up at all, I can tell the team it'll be back up soon and take notice. To be fair I play jungle more often than not so maybe it's due to that, but I find that when I lane I'm frequently calling dragon notifications on spawn times because my jungler isn't - which makes me feel like junglers now no longer take responsibility for that action (before timers I rarely called times when I was laning and my junglers didn't always call them but they often did).