Sure pios have other things to do than repairing but when something needs repairing you can be pretty sure your grenadiers ain´t gonna do it. So I wouldn´t say it´s a bad idea to train maybe second one when you have heavy tanks on the field. Also, while the actual process of repairing might take some time, the order can be given very fast even to multiple vehicles via shift click.
Hehe, I am fully aware of shift clicking, but you speak like your tanks are perfectly lined up at your base every time they need repair, ignoring for instance damaged engine. Having two or more pios is a waste of popcap and resources, and face it, its an advantage to have several ways to repair units. You can also repair at a much faster rate. Thats why its a doctrinal ability for OKW. The weaknesses of fielding heavies I've explained in detail in this thread.
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