What is the motivation for the use of 把 or 将？ Chinese is a language without case markings on words, but it does make some steps in that direction. It sometimes marks what in some languages would be an accusative or oblique case noun with 把/将 and leaves it in front of the verb. What is the motivation for this? Speech rhythm? Effect? Emphasis? I believed all such sentences could be rewritten without 把, but maybe this one can't?

那时戏剧界有一种很不好地风气，就是把戏剧当成一种时髦。

As far as I know, 当成 means  'regard as, treat as, consider as' if a person is mentioned, or 'regarded as, treated as, considered as' if a person is not mentioned, which implies a third party regarding 1 object as something else, here '戏剧‘ as ‘时髦’

Therefore, '，就是把戏剧当成一种时髦。' is a passive construction, neither '戏剧‘ nor '时髦‘ are regarding anything, a person is doing that.

Theatre was regarded as a vogue.

Why would you say 1.'，就是把戏剧当成一种时髦。' rather than 2.‘，就是戏剧当成一种时髦。‘ We know 'theatre' cannot regard things, people do that.

Is 2. wrong?
 Can I rewrite 3. 你把我当成什么人? as '你当成我（为）什么人？ In 3. we see the 'regarder', 你， whereas in 1. the 'regarder' is not mentioned.

It is said that 把 marks the 'object' of a verb, in which case ，'把戏剧当成一种时髦' has 2 objects and no subject, so to speak, namely '戏剧‘ and '时髦‘。 The person or persons 'regarding' it are not mentioned.

However, even for me as a 老外， ‘就是当成戏剧一种时髦’ sounds terrible.
Maybe '就是人们当成戏剧为一种时髦‘ is acceptable.

Rule: When the regarder is not mentioned, fill the gap before the verb with 把X， where X is the thing regarded. 

Does that account for this syntax??
it's a simulation of english grammar, after may fourth movement.

就是把戲劇當成一種時髦 is roughly:

that's, (就是) to treat (把) drama (戲劇) as (當成) a (一種) trendy things (時髦)

imo, modern days 白話文 is un-xxxx-able; using western (english?) grammar to write, or analyse written chinese is, well .... i'm not dare to say it openly.

personally, i read &amp; write literary chinese as far as i can.

edited.

那時戲劇界有一種很不好地風氣，就是把戲劇當成一種時髦。


this sentence in literary chinese, it could be (there're ways to express the idea, here's just one of the attempt) written as:

時風澆薄﹒以戲為髦

that, the structure of "把 a 當成 b" is represented as "以ａ為ｂ"; so, the character "把" was seldom used in such situation.

But this use of 把 is not found in old Chinese texts from say 1000 years ago? 


no, 把 was not used in such situation in the past; based on my experience.