I think it's because originally the ka was a professional and so that people like me who only do aiki 2-4 times a week could not consider themselves to be one. More like an amateur or an enthusiast. Same way some people don't like to say aikidoist because it implies a profession like a dentist, geologist, scientist but i think that is feckin stupid because : numerologist, philatelist, scientologist... none of which need be professions. To be honest, most of them are most uncool. Apparently though, the "ka" has been chilled out in definition recently.. so it's ok. thanks!