FYI, exact match does override broad match. It's often useful to include multiple match types in a single ad group, i.e.:

[blue widgets]
"blue widgets"
blue widgets

and often at different bids. You know exactly what you're buying with the exact match, so bid higher on that one. With phrase match you have some idea what you're buying so bid a bit lower. With broad matches actually implemented as expanded matches, you have no idea what you'll actually be stuck paying for so bid *way* lower.

Oh, here's the relevant part of the help page so you know I'm not just making this up:

"The more restrictive match type will always trigger the ad, regardless of CPC bids. For instance, if the broad-matched keyword apple and the exact-matched keyword apple both existed in your account, the exact match would always trigger an ad."

Last thought - when using multiple match types in a single ad group, you can learn quite a bit from the impressions data. ;-)

In theory, this is all gravy. In practice, however, the rule of the most restrictive match type triggering the ad can be overridden if you have pre-existing history with a less-restrictive match type.

For example, let's say you have a broad match bid on "red widgets" and you establish good history and a good quality score with that. Combing your reports/logs and creating phrase and exact match bids on every variation of "red widgets" does not necessarily mean these new keywords will cannibalize a proportional percentage of your broad match keyword's traffic. There's a strong chance that your broad match keyword(s) will continue to capture the lion's share of the impressions, regardless of how many phrase/exact match keywords you've added.