What do you hope to achieve by doing this? Assuming that your equipment is maintained and in working condition, you're replacing a dependable tool with an untested tool that offers nothing more than a built-in meter (?).
I don't know how "good" the builtin meter is on the M6, nor do I know if anyone can repair/replace it should something go wrong. (I do know that it's becoming more difficult to find certain replacement parts for the M4-P and thus (I assume) the same will apply to the M6.)
I don't know how many M6's are susceptible to the "viewfinder flare" issue that I've heard about on this forum, but, again, I don't know if this is a problem with the M4-P. I own and use two M4-P bodies almost daily and have had no "focussing flare" arise in varied lighting conditions.
I do know how to use several hand-held meters that are perfectly acceptable for everything from controlled lighting (flash units, Norman units, etc.) to almost every likely working scenario. By the way, some of these units are quite portable. I would go even further and say that I prefer a good quality hand-held light meter to any built-in meter that I've used on the popular SLRs during the 1970's era.
I guess, at the end of the day, this is a personal decision; you have to choose what's going to work for what you do.