My wife absolutely hates the character and she pointed out why the other night when the episodes were on...
When they're first attacked by the Borg and Shelby comes forward and starts ordering Data to remodulate the phasers, once they break free she has this smug ass look on her face that gives you the impression that she thinks she's the only one who knows anything.
Not surprisingly, I don't interpret the scene this way.
I see Shelby's reaction as a major personal victory for her. For some undisclosed period of time, Ms. Shelby worked at some Starfleet Command facility or team responsible for anti-Borg defense research. This we know from Admiral Hanson. But since the Borg had only just returned to Federation space since the J-25 incident, this was the very first time Shelby's work could be put to the test. She thought on her feet and her orders to Mr. Data produced results.
Mr. Data, carrying out Ms. Shelby's orders, saved the ship.
I personally didn't find the character professional or sexy. I thought it was very unprofessional for her to be trying to push for the XO job while Riker was still there and going over Riker's head when he didn't agree with her plan to separate the saucer.
There is only an initial trace of resistance on Riker's part to saucer separation. He simply expressed doubts about it; he never completely dismissed the idea. In fact, Riker later used Shelby's idea (along with a few of his own guerilla tactics) near Wolfe 359 to successfully rescue Picard. Riker made it clear in the turbolift that his real problem was going to the captain without him. I agree that was an unprofessional moment for Ms. Shelby. So was Riker's uncharacteristic handling of her.
She was an arrogant junior officer who was use to getting her way because she worked for an admiral back at Starfleet.
Her "arrogant" pushiness was apparently a driving force behind the anti-Borg operation that impressed a Starfleet admiral. And while there is no direct evidence of this, it is a pretty safe conclusion that this same program, under Shelby's administration, ultimately resulted in highly successful anti-Borg defenses such as Sisko's Defiant.
If you want to see a dramatic portrayal of the "real McCoy" in a pushy program administrator that was responsible for saving the lives of real-life astronauts, check out the outstanding APOLLO 13