Besides ARM is much faster. When playing Germany you will need the speed to encircle your enemies. Using MEC once they are researched will maintain the speed AND still give you combined arms bonus.
I always go pure armor brigaded with SP arty. They are absolutely unmatched as spearhead formations. Back them up with a couple corps of MOT and you have a winning combination.
the combined arms bonus gives your MOT formations just enough punch to drive through the enemy.
Yeah, but I only build 6 panzerarmee total, so it's not that bad. TC always sucks when playing as the Germans when you go into Russia.It's true that 3x armor is the most powerful corps, but it's murder on your TC. If you use 2XArmor and 1xMot, or even vice versa, you can stretch out your tc a little more, while the combined arms bonus gives your MOT formations just enough punch to drive through the enemy.
As Germany, going to war early on, I tend to build corps of ARM/MOT/MOT. If I play UK, USA, or USSR, which can all delay the land war to '41 at least, ARM/MEC/MOT is a better choice.