Peter A Goldsbury
Well, there is a seeming arbitrariness about the matter.
Aikidouka is a hybrid term in Japanese, not in common use. Like 柔道家 juudouka, it has come to be accepted to denote someone who practices aikido as a regular pastime. KA seems to be preferred to other suffixes, like SHA (者, meaning person).
However, someone who trained hard at bujutsu, like Takada Sokaku, wandering round Japan doing 武者修行 musha-shugyou, was called a 修行者 shugyou-sha, not a shugyou-ka. Why? I do not really know.
On my computer, 合気道者 comes out readily when I type in ‘aikidousha', but 合気道家 never appears when I type in ‘aikidouka'. With judo, the situation is reversed: 柔道家 (juudouka) is clearly accepted by my computer kanji dictionary, but not ‘aikidouka'.
Since ‘aikido' has become anglicized, there is a similar arbitrariness about what to call someone who practices aikido. The name has to sound ‘right' in English.
This kind of issue is partly what makes the study of Japanese kanji so interesting.
In German, the direct equivalent to aikidoist would be something like Aikidoer (in analogy to Fechter, Ringer etc.).
But, in the early sixties of the last century, when aikido came to Germany, many early aikidoists were equally judoists and members of the DJB (Deutscher Judo Bund); so they probably took advantage of the superficial analogy between "Aikidoka" and "Judoka", this in common use for a judo-player then and now.
Like in many other countries, the term "Aikidoka" is now widely used and apparently has evolved into a kind of appropriate one-word description, singular or plural, without (anybody ever caring about) its deeper meanings in Japanese.