Lose or damage your external supply before a gig and your SOL.
Barrel plugs are non-universal and non-standard. There are 20+ variations of the plug end (both size and polarity). Its not like you can just pick up a replacement power supply in your local hardware store.
Lose your IEC cable and you can pretty much find a replacement cable anywhere. Just about Every computer, monitor, TV, cash registers, etc. etc, that uses a removable plug has one (meaning it can be borrowed if needed).