When I started aikido I had a Japanese sensei and when I called myself an aikidoka he said that I could call myself what I wished but I had to understand that the '-ka' means that (1) I devote my life to the study of aikido, (2) I make my living from aikido, and (3) I am prepared to die at this moment if challenged by anyone for any reason.
With the proviso that these are purely my own opinions, and thus subject to change if someone convinces me I'm wrong..
(1) I agree with, absolutely. But then I don't believe you can 'play' at learning Aikido - though I imagine you could 'play' at turning up to practice...
(2) I don't think is so important - not when you take onboard everything that Aikido is. I mean it's not like calling yourself a Chemist because you read Nature during rest breaks when you're driving a truck... (Refering back to another thread, I do now agree it's a good idea to earn a living from teaching Aikido - so you can study full-time. I just don't believe you're necessarily less dedicated because you don't have that opportunity...)
(3) Is poppycock... Personally I'm prepared to die, at this very moment, to protect a friend or loved one. Without question. Anything else is subject to terms and conditions - most of which involve it not being a 'matter of principle', or stupid!!...
(N.B. I should clarify that by matter of principle I mean something akin to 'arguments', rather than, for example, stopping someone hurting people...)
... From the point of view of having to explain what Aikidoka is everytime I use the term, I'm probably in favour of 'I study Aikido' (Saying "I follow the way of harmony" will probably have people thinking you're a cult-member