Many search marketers will recognise the scenario in which a (potential client) very proudly says they have followed all the SEO-rules in order. And then the first thing they mention is that all the meta tag keywords are there.
A second scenario will also be recognisable, when taking a look in the code of a website you find tens, maybe even hundreds of keywords stuffed onto the page, just to get Google to 'find the page' based on the keywords.
Many times it then is difficult to convince your potential client this is all useless. Now Google has made it 'official': Google doesn't use the "keywords" meta tag in their web search ranking.
In the video below Matt Cutts talks about the issue.