Is body marking a requirement?

No, it's not a requirement, although they will say it is - and it actually might be in the rules,. haven't checked, but no one will stop you on the race or (probably ) DQ you (if you do get DQ'd don't come crying to me tho!   ) .   The sharpie always gets pretty much washed/rubbed off by my wetsuit by the time the swim is done.   You are recognized by your number on the bike and race belt on the run.

I have a friend that often just skips it and never gets marked.

I wouldn't even mark the swim cap unless the race is marking them for you (some, very few, do).  Some (again, very few) mark your hand

IMO it's really more of a ceremonial/part of the experience thing, like I said my markings are pretty much gone by the time the wetsuit is off.   And to have the number on your arm and get asked what it's for in line at In n Out after the race  

In a few HIM's I've done, they used the blocky, stamp on numbers and after a day in the sun left my number etched on my arms until my tan evened back out.

I always figured that they were more for bragging rights afterward than anything else.

That and for the "official" race photographer be able to identify you so you can pay exorbitant prices for your race pictures.

Mark

Depends on the race.  Pretty sure they all say required.  A lot of the small races I do are not real strict about it.  Big races are all very strict.  Another thing to be mindful of beyond what was mentioned like being DQ'd, some races match your body mark, bid and bike number before letting you out of transition.

Another issue is if you use white arm coolers or calf sleeves.  The ink will get in the fabric and won't ever come out and will just be a big black smudge.   Makes the gear look like cr@p

I must be one of the few people it never rubs off.  That's why I don't like marking.  Especially in races when you need to get marked the day before for bike drop off.  I race a lot at the Jersey shore.  I have little kids so day before race and post race I am usually out doing something with them.  I feel like a complete tool at the beach with body marking.  If it only took me a few mins to clean it off I wouldn't mind.  I am usually scrubbing with rubbing alcohol or nail polish and will go to a scour pad to get it off. 

It must be something to do with skin type.  I look at other folks leaving T1 and their marks look fresh as daisies.  Mine looks like a Rorschach blot....

ETA I also don't always follow the no sunscreen rule prior to marking so that might contribute but happens at shorter races where I don't use sunscreen depending on conditions

I'd have to look but I'd guess I've done maybe 30-40 races.  Never has it rubbed off !  I've seen others have it fading during a race but not mine.  Sharpie, whatever other marker gets used sometimes, tattoo's , you name it.  Sharpie I can usually get off with fresh nail polish remover and a nail brush.  Turns my skin red like a rash.  I did a HIM last year and had the tattoos for at least a week.  That's with scrubbing it daily

Those tattoos are disgusting when they start coming off.  Hate em

I literally haven't been body marked in six years.  I had marker bleed ruin a tri suit years ago and vowed to never again be marked.  I just walk right by body marking and nobody has ever said a word.