What PLC are you looking at Boyan?
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Sometimes you have to use a PLC, and other times it just makes life a bit easier.
Mach 3 only has a 10Hz refresh rate for Macros, which means any sort of ATC with critical timing can't be done directly. I used a PLC for the turret on my lathe, as the locking mechanism timing needs less than 50ms between sensing an input change and an output being activated, which wouldn't be possible with a Mach 3 macro.
Mach 4 does have a faster refresh rate, but I'm not sure how fast it can react.
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However most ATCs on a Mill aren't that time critical. It's a just a relatively simple process cycle, with the biggest problem likely to be how long it takes. It's just a case of working out the required sequence of sensors and outputs.