Update: Microsoft is finally fixing this. See comments below.
This post explains how the engines understand keyword match types in PPC campaigns and why you have to use all three match types in AdCenter.
Both AdCenter and Google have three major keyword match types: broad, phrase and exact. Each match type typically performs differently. Based on their performance you may run only some types and pause the other. For example, quite often the broad match brings less relevant impressions, has higher CPC and lower quality score; therefore it usually gets paused first. Generally, is a straight forward process: find the bad keyword and pause it.
However, there is a key difference in the way search engines see these match types. In Google each keyword-match type pair has a unique ID (number):
In AdWords, when you pause a match type, you are only pausing a keyword ID associated with that match type. Other match types will continue to run unaffected.
In adCenter, on the other hand, all match types are associated with one Keyword ID:
In adCenter, when you pause a keyword type, you are pausing a keyword ID that associated with all other match types, and you are telling the system to pause all the keyword match types. Be careful with how you manage your keywords in adCenter! Before you pause a low performing keyword type, be aware that you will be pausing other match types of the same keyword and maybe loosing performance there. Basically, it is everything or nothing for a keyword.
Obviously, this is a flaw in the system and adCenter is aware and working on it. The solution should be coming sometime in 6-8 months.