Q:

Slugs: are human-friendly or SEO-friendly URLs better?

Well, I have a co-worker (actually my superior) who often sets his articles' slugs to expected / relevant search terms such as:
keywordA-keywordB-keywordC-keywordD-keywordE

I usually make mine closer to the article's title, e.g.:
robber-robs-XYZ-bank-10-million

He claims placing only the expected search terms as the slug led to better SEO and that SEO is more important than a human-friendly URL.
What do you think of the balance between human-friendly and SEO-friendly URLs and does littering your slugs with a few expected keywords actually lead to better SEO? Lastly, what is the recommended practice?

A:

Your co-worker's method looks suspiciously like keyword stuffing and may not actually be relevant if the content doesn't contain those keywords. That's not to say URLs should be devoid of keywords, I just think it should be planned better and make sense to humans too otherwise it could just look really "spammy". 
I don't completely disagree with him if it's done right, except for this point:

SEO is more important than a human friendly url

I strongly disagree and will forever believe that regardless of all the tips and tricks developers and marketers can employee (above and beyond just having good content), URLs should make sense at a glance if it's achievable. Having URLs that are close to the title of the post / page definitely makes more sense to me and since the same words will also be present in content, it'll be good for SEO as a by-product.

Edit to answer your edit
One recommended practice is to try to curtail the length of your URLs to 10 words or less, since more will just have less weight when being evaluated by the search algorithm. 

A:

I'm kind of surprised that no one said that human friendly URLs are search engine friendly URLs. Human friendly URLs usually are naturally:

going to contain keywords related to the content
going to read naturally and not look like a two year old wrote it
will usually, but not always, be short and to the point

You really shouldn't need to be considering the search engines when creating your URLs because if you write them for human beings they really are search engine friendly already.

