Is it safe to use with the battery disconnected?

Lets look at it this way:

• If you dislodge the power cord, what you are working on will be corrupted so the battery is acting like a UPS protecting your work.
• Your system was designed to be a sealed system
• The space internally were the connector is located is not big enough for the connector to be disconnected without long term damage with the cover off or on.
• Constant reconnecting will wear the connection and if you are not careful you could also damage your system if the unit was not fully off (sleep mode) when you altered things.

Bottom line here: leave it alone it was designed to be used this way.