The XK has a 160 amp alternator, so generator power is a non issue. but, if you want to run a 'fridge, high power stereo, etc. without running the engine, then a 2nd battery makes sense. That way, battery two runs down, and you still have a starting battery.
A lot of cop cars have a 2nd battery in the trunk to run radio equipment and lights. We use one of these to connect the 2nd battery. This device will allow the 2nd battery to go dead, yet you start with an untouched main battery. Or... if the main goes dead, you have an auxiliary to help start up with.