Thank you for clarifying that myth - I won’t exercise the batteries as much as I used to do! 
Still, it seems that the “unspoken cost” could be the shorter life-span of the MBP’s battery, since it’s always charging.
I’m curious because I need a similar solution and CalDigit’s seem to be very popular. I wonder if I’m exagerating its possible impact in battery life… 
Recent versions of MacOS prevent this by halting charging at around 80% in such circumstances. I have a MacBook Pro that’s always plugged in to a similar dock and it hasn’t been fully charged in months. It also doesn’t discharge by any appreciable amount. In over two years of nearly continuous use, its battery discharge/recharge cycle count is only 34. (Had I known about the impending pandemic and the resulting work from home situation, I would probably have purchased an iMac instead)
Just chiming in to say I agree with @ACautionaryTale. The impact on modern batteries is minimal if any. Also, even if it were (and it’s not), I’d still do it. I bought the laptop to use it in the way that serves me best. I wouldn’t be inclined to let a potential future need to replace the battery create daily hassle in my workflow.
This CalDigit dock has created one of the best setups I’ve ever had. It’s making me think I won’t buy the new iMac when it comes out. But I will 
I don’t see this charging behavior on my M1 MacBook Air, but I wonder if it is because I usually unplug it when it reaches an 80 percent charge? Maybe if I left it plugged in all the time, it would eventually begin halting the charging process at around 80 percent?
If you do leave it plugged in all the time, I recommend you check for beginning signs of battery swelling a month before your three-year Apple Care Plus runs out so that you have time to send it back to Apple and get a new battery under warranty, if needed.
I don’t (as) consistently see this behaviour with my “around the house/travel” notebook either. It’s nearly always plugged in while in use, but occasionally sees time on battery power. It usually charges to 80% and remains there, but sometimes it goes up to 100%.
I think that what’s happening is that it’s trying to anticipate the times that I will need it at 100% based on my usage patterns, which are non-patters for me. I do know that it does try to learn your patterns and charge accordingly, so if you spend a lot of time using your computer on on battery power then it should avoid halting the charging at 80%.
So you think its algorithm says something like “go to 100% as long as there is little history of being plugged in at maximum charge?” I hadn’t thought of it that way, but it makes a certain amount of sense now that you mention it.
I think the algorithm is slightly more complicated, and I think that Apple hasn’t quite mastered it yet. I honestly wish they would have an option for 100% manual override in that it would have a setting to never charge past 80% unless I say so.
Purchase a new, full Mac keyboard, it will give you access to TouchID. Just a thought.