I fitted one to my Caterham when I built it a few years ago, and still consider it a worthwhile mod. I haven't done the Morgan - yet, but will probably get around to it. Not only is it a good thief deterent, although the 'key' is standard, it is not something most thieves will have on them, more importantly, by disconnecting the battery, it will stop it discharging, during long off road periods, which cars like this enevitably have, particularly if an imobilizer is fitted. There is also the safety aspect of being able to disconect the battery quickly in the event of an incident.
BTW, it won't fall out once turned on as there is a bayonet type fitting in it, a bit like a light bulb. You can leave the key in and forget about it if you want, it won't do any harm, but I dn't find mine at all fiddly, although it will depend on where you have sited it I suppose.
Make sure to go for the slightly more expensive Motor Sport fitting with a 'gizmo' that will drain current from the alternator if the engine has to be turned off with the big red key, instead of the ignition key. If this happens without the 'gizmo' then the alternator will fry. Look in the Demon Tweeks, Burtons or other good Motor Sport suppliers catalogue. From memory, about £15 - £20 should cover it.
