There is an interesting paid search thread over at WebmasterWorld on the topic of using match types. For newbies, match types basically give you the ability to tell how specific or broad you want Google to match your keywords. So if you want Google to show your ad for the keyword phrase blue widget, and variations of it, like big widget that is blue, you use a specific match type. For more on how match types work, see this help document.
That being said, Google is now recommending that you do not list out multiple match types for keyword phrases. Typically, an advertiser might target several keywords, but be very specific on those keywords. So what Google seems to not want you to do these days is add all of the following to your keyword list, but instead decide to go all broad or all narrow.
Example: 'blue widget' [blue widget] [widget blue] [big blue widgets] and so on
Senior member, bcc1234, said he got word from Google that "they discourage the use of the same keyword with multiple match types." He explained, that "instead of triples of all keywords, they want advertisers to go from "broad" to "narrow" (in their words)." He thinks this makes sense, why?
It actually makes sense to some degree. But I'm guessing they are trying to cut down on the total number of keywords in their system. I imagine with millions of advertisers, and lots of them uploading hundreds of thousands of keywords in dozens of accounts, it really becomes a saleability issue.
Forum discussion at WebmasterWorld.