The size of the screen doesn't matter, only the resolution.
The monitor have it's own power source. All the MBP do is sending information trough DVI cable. Yes, in itself it does take more power from the MBP, but it is a totally NORMAL utilisation. Also, having two screen mean more pixels so more calculation for the MBP, but again, it's a computer, it is a giant calculator, it is totally normal.
As for the battery, DO NOT use your MBP without it, because the MBP is programmed to run at half it's capacity when running without the battery, even when the charger is plugged.
The reason this guy is having 10 minute of battery life is not because of an external monitor but simply because he obviously always use is MBP with the charger on. The electron NEED to move into a battery to maintain a good "health". Battery life is more affected by time it stays unused (or fully charged with the charger) than cycle. USE your battery, don't fear using it for two hours at starbuck or going into your living room without the charger. The worst thing to do is always looking for a place to plug the computer. The battery is made to be used.
Of course, you can keep the computer plugged in even if the charge is at 100%, just use the battery from time to time. Also, do a Calibration every 1 or 2 months like Apple reccomand (type "Battery calibration" in the HELP search of Mac OS X)