In a way - its about the maximum power consumption. If you have a small laptop then the battery will charge ok while you use the laptop. On my laptop (thinkpad W500) I had to turn off the wireless and bluetooth otherwise the power consumption was too much and the power switched off every 45 seconds or so.
One alternative (if you don't want to remove the battery and risk instant shutdown should the power fail) would be to use the power to charge your laptop, then disconnect the power, startup your laptop. use it for a while until the battery runs out. close it and charge it. and repeat.
I got a new power adapter for my laptop from lenovo, you can plug a mains cable into it or a 12v cable with empower plug at the end, so I only have to travel with one charger (although it's slightly bigger than the standard charger).