Why should someone learning Chinese as a second language learn 成语 idioms?
Idioms and classical phrases are important because they are parts of the modern Chinese.
The obvious benefit of using idioms, is it can greatly reduce the length of our sentences
Look at the following example:
跟他做交易真是[與虎謀皮 ]
The idiom 與虎謀皮 in this sentence, expresess several ideas

It is dangerous to deal with him

He is untrustworthy

He is a vicious character

It is foolish to expect good result


If you don't use the idiom 與虎謀皮, you have to use extra words to express all the ideas above. The sentence would become something like this: &quot;他不講信義，手段兇狠, 跟他做交易是一個十分危險而且愚蠢的行為&quot; (He is vicious and untrustworthy, dealing with him is a very dangerous and foolish act)
Just from reading  the four characters 與虎謀皮 , we know the person you are dealing with is 不講信義(can't be trusted)，手段兇狠 (vicious) and the deal itself is 危險 (dangerous) and  愚蠢 (foolish)
I would say learning idioms is as important as learning anything else in one's study of Chinese. Idioms are refined short cuts to express complex ideas in simple and comprehensive way
Origin of 與虎謀皮: http://www.cantonese.sheik.co.uk/phorum/read.php?14,131767,131767#msg-131767