Hm, interesting. So it's not m/wa class... Maybe because doctors aren't people but demi-gods :)
"Daktari" is a Ma class noun, just because (I think) it is a loanword from English "doctor":
I just started the Ji-/Ma- class and in the description it says that most occupations are in this class. So probably this is why... Other words from English seem to be in the N-/N- class (without any plural), but "daktari" apparently is an exception.
I read somewhere that loanwords used to routinely be put in the Ma- class but in modern times tend to go into the N- class, and a lot of nouns are drifting from the Ma- class into the N- class, such as rafiki.
@BenUserName: Swahili is such an interesting language in this regard... Extremeley fluid. I love how it constantly assimilates the most diverse of influences!