If you want to a) have a huge external device connected all the time, b) destroy your battery with constant charge cycles so badly that you need to replace it every few months, and c) want to be unable to charge other devices from your laptop.
The question I replied to was about charging other devices from laptop. You can charge your other devices in your bag with battery pack, not dangling from your laptop while you're charging them.
Having a cable go from my laptop to a battery in my bag isn't the biggest issue, even on my lap, but maybe things are different for your use case. Granted, since my laptop lasts >12 hours, I never really worry about battery life too much anymore anyway.
