Are there reasons not to use always the max level of assistance? I'm using an electric bicycle to go to work. My goals are:


Not to arrive late
Not to sweat (too much)


My bike, like most e-bikes I guess, let me choose the level of electric assistance that I wish. I understand that the range is longer with a lower level of electric assistance. But in my case, this is not relevant. My battery can take the whole commute regardless of the level of assistance.

Is there any reason I should not use always the maximum level of assistance? For instance battery or motor damages/heating/...?
Assuming you have a good quality electric assist bike from a major manufacture. The manufacturer provided the max assist setting, so they intend for it to be used. It's a safe bet the bike is built to take the max assist torque without sustaining damage or accelerated wear. If the manufacturer believes sustained use of max assist will affect the bike, it will say so in the manual, so check that. 

The one thing I'd be mindful of is battery life. General advice for laptops, phones and other portable devices is that deep discharge and heat degrade batteries and shorten their life-spans. If your commute is short, and the battery capacity is adequate for max assist both ways, that should not be a problem. I'm not sure about how discharge rates affect battery life - if I find info on this I'll update this answer.

Otherwise I say go ahead and use max assist. Yes, more torque and speed will wear bearings, chain, etc. proportionally faster, but using the assist is what you bought the bike for, right?