How bad is always having my MacBook Pro 2017 connected to electric supply

I wouldn't worry about it. But it's a good idea to at least use it just off the battery every once in a while. Maybe take it somewhere for a few minutes. For whatever reason that seems to reduce the chance of the battery swelling.

http://www.apple.com/batteries/maximizing-performance/

Re: Powerful transformer vs battery life. OK to use?

You need to use the battery from time to time because it needs the exercise to stay in good condition. Use it for a couple of hours here and there and recharge it when it gets down to 10 - 15%.