Young skywalker I think you are getting warm! A personal log book forms a record of your personal experience and it has many uses, it can impress friends, employers and the CAA etc. It also has the function of proving your experience in the event of an accident.
For instance lets say in theory that two identical pilots had the same serious accident in an Aztec. Both pilots had previously had 10 hours experience of multi engine aircraft. One pilot however had a 1000 hrs P2 time on a twin not required to have a co pilot and had flown leg and leg about with the captain but that pilot following advice on this forum had not logged any of this flying. Now ask yourself the question that if the AIIB looked at both of those pilots log books would they assume that both pilots only had 10 hours experience in multi-engine aircaft and would that be the case and a true reflection of each pilots experience?
Another reason for I would advise you to log all flying is because in years to come you may want to look back and reminice about the aircraft you were in,the people you flew alongside and the places you visited. My son, much to my annoyance sometimes, gets my log books and asks me about flights I made 30 years ago and I see that some of those were P2 in single engine aircaft were I flew as a safety pilot while another pilot flew an IFR cross country. I have never counted any of those hours (or even totalled them) for anything but they form a personal record of my flying experience as a non handling pilot and one that i am very glad to look back on, so take no notice of the barrack room lawyers, you put what you like in your log book its your property not the CAA's!.
Its a long time since I flew King Airs but my FOs at the time counted their P2 time under an agreed scheme with the CAA, I seem to remember. The best place to seek authoritive information on the sublject is from the CAA themselves but always make sure they put it in writing. Over the years the CAA have granted dispensations to quite a lot of different bits and pieces of legislation (when there are legitimate and valid reasons) but the first requirment of couse is to ask!