Yes I do, even when I know when I'll get home, plans can change.

A timer also can't tell me if my AC is actually on, may fail in the case of a power outage or I may forget to set it, as such using a timer doesn't provide a meaningful improvement over just turning on the AC when I get home and tolerating the heat for half and hour or so.

In the end, a timer has no feedback, so a closed loop system will always be better. I've been burned numerous times by open-loop systems in home automation, so I don't do that anymore. That's the second reason why I spend a lot more effort and money on sensors rather than actuators (the first reason being that graphs are fun).