Your post got me thinking about this and from searching on google spotted a few other discussions on this which are shown below.
From these discussions it appears people have mixed views on this. Below are some of the Fors and Againsts I can think of.
It means you can use different negatives for different match types which prevents your broad/phrase match terms competing with exact match terms by adding negative phrase or negative exact match terms. EG - If you use the term BUY WIDGET in one ad group as broad and as exact in another ad group then you can could add -[BUY WIDGET] to the ad group with the broad match in to prevent the broad match triggering the ad for exact searches on the term.
Allows you to use different ad text for different match types eg with broad match you dont always know the exact seach query that the user has searched on and so by using more specific ad text for this match type you can try and reduce unwanted clicks by making it clearer from the beginning what you are advertising. With exact match you know exaclty what the user's search query was and so you may wish to make the text more encouraging for users to click on. This may also help improve quality score as you may find the different match types perform better with different ad texts.
It doesnt make it as easy to compare the different match types perfomance against each other. If they are in the same ad group then you can just filter the ad gorup to compare stats whereas this would not be as easy if you have them in seperate ad groups as you will.
It means you have more data to manage eg 2 or 3 times as many ad groups and ad texts to look after and monitor.
Personally I think it may well be worth experimenting with a small portion of your adwords campaign to see whether it does benefit you or not - if it does help improve your terms qulaity score then it it could well justify the increased amount of data you will have to manage once you split up the whole campaign.
[edited by: Jonerock1t at 2:30 pm (utc) on Sep. 14, 2007]