I like most don't see the need for keyless technology. I am quite happy with an old school key. It works. It is harder to nick an old car now than a newer one it seems. You can certainly lock the key in the XJ....I've done it! Luckily I was at home and had the spare handy.
Worst one was locking the SL key in the boot when I was popping the trickle charge on. You can open the boot with the key and it stays open until you close the boot. It doesn't open the two front doors as you haven't unlocked the car. When you close the boot and that noise triggers your brain to realise.....pants....they key is in the boot isn't it! Even worse thing was I ordered a spare 2nd key for the SL 2 years ago.....still haven't got it yet as you can't buy them from Mercedes. On a list until a batch is made. Luckily I could order a new blade from the V5 and a form and got it in a week to be able to open the boot to retrieve the key again!!
I never open the boot only on any car now and will open the car at the same time. Annoying thing is I always pop and keep the key in my jacket or fleece (classy me!) when working in or around the cars for this reason. Lack of attention that time and paid the price