The dashes are a grammar construction indicating that what is between them can be completely omitted without changing the sentence to any degree.

The article is intended for "professional," therefore it should be "a".

In my opinion, the "- in our opinion -" is in the wrong place anyhow.

It probably reads better if written as,

"He performed his duties - in our opinion - in a professional manner"

or

"He performed his duties in a professional manner - in our opinion (, at least)."

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Note added at 2002-10-23 09:26:59 (GMT)
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Regarding your added Note:

Without rewording... you might get a \"minus\" added along side of your grade. But leaving it as \"an\" might get an entire letter grade knocked off. Even leaving the sentence unedited, I would still propose \"a\". Grammatically speaking (as a check), if you can\'t remove \"in our opinion\" from the sentence without having to make a bunch of other edits, then what you originally wrote was probably wrong.

Dashes account for sentences like \"The dog - by the way, my friend Karen just bought a dog too - is spotted and has floppy ears.\" This sentence is grammatically correct.

Leaving the \"an\" in your sentence is like saying \"The dog - by the way, my friend Karen just bought a dog too - that\'s spotted and has floppy ears.\" This sentence, of course, is _not_ grammatically correct.

Why? try the trick... see what you get.



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Note added at 2002-10-23 09:45:28 (GMT)
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You should probably check out the function of \"asides\" vs \"parenthetical phrases\" in a grammar book.

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Note added at 2002-10-23 20:27:52 (GMT)
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Just another note... If you do not pause where the dashes appear in the text, then you\'re probably not doing the punctuation justice when you read it aloud. A dash is considered a VERY strong punctuation marker to isolate that bit of information from the rest of the sentence. If you run it all together, then there would be no point in using any punctuation at all.

As I mentioned to Christopher Crockett, punctuation governs the way written text is to be spoken and understood. If you\'ve taken a Performance of Literature course, it would probably be a little clearer how the punctuation plays a role in how this sentence (in original form) is to be read. If I had a way to record the two options and upload the files, it would be 100% clear.

Furthermore -- regarding the cbolton comment about awkward sounding text, and people writing the way \"they\" speak -- this sort of writing may be suitable for a chat room on the internet or in a buddy-buddy conversation with a friend. From my standpoint however, spoken speech has always been more accepting of wrong or improper grammar usage. This is primarily the reason why issues such as this even come up.