Why Can't You Take Plant B All the Time?

Say you only have sex 2-3 times a month, and money is not an issue, why can't you just take Plan B after each encounter instead of having to take a birth control pill every day.?
Update: Plan B is 95% effective if taken soon. Birth control is 99% effective and can get screwy if you miss just one day or more...That's only 4% difference! That's nothing!
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