I don't know...I think "I want a secretary" is perfectly fine to say in the business world or in general company, as long as you don't say it with any sort of inappropriate innuendo. If you're just saying to your boss or co-workers, or friends or family or whomever, that you want a secretary (because presumably, you're overworked, or maybe just lazy), I don't think anybody would react negatively.
I think the difference is whether you are talking about a specific person or not.
If you say I am a secretary, you don't use the 'en' because you are talking about a specific person (me). Here, the word secretary is describing "my job title".
If you say I want a secretary, you are not talking about a specific person. Here, the word secretary is describing "a person who does that type of job".
Not sure if I've explained that well, but hope it makes sense.
You've got it. "Jag är sekreterare". It's an identity of the subject (profession, religion, political party, nationality, etc). "Jag vill ha en sekreterare." It's something the subject wants.
A little off-topic, but when you use the "identity" word more figuratively, you do use the article:
- Han är clown. He is a clown = It's his job.
- Han är en clown. He is a clown = He behaves like a clown.