Highly useful. Let’s take a look at the Band of Nine:

THE OLD MOTHER: A pirate queen.
SAMARRO SAAN, THE LAST VALYRIAN: A notorious pirate from a notorious family of pirates from Lys, with the blood of Valyria in his veins.
XHOBAR QHOQUA, THE EBON PRINCE: An exile prince from the Summer Isle, he had found his fortunes in the Disputed Lands and led a sellsword company.
LIOMOND LASHARE, THE LORD OF BATTLES: A famed sellsword captain.
SPOTTED TOM THE BUTCHER: Hailing from Westeros, he was captain of a free company in the Disputed Lands.
SER DERRICK FOSSOWAY, THE BAD APPLE: An exile from Westeros, and a knight with a black reputation.
NINE EYES: Captain of the Jolly Fellows.
ALEQUO ADARYS, THE SILVERTONGUE: A Tyroshi merchant prince who was wealthy and ambitious.
MAELYS BLACKFYRE, THE MONSTROUS: Captain of the Golden Company

Of the Nine, three are pirates who could provide the Golden Company with badly-needed naval support for their amphibious invasion. (Remember, the Golden Company is a land-based mercenary army that needs a lot of ships to transport itself by sea. Hence the whole malarky they get into with Volantis in ADWD.)

Three or four (depending on where you put Derick Fossoway) of them are other sellsword company leaders, no doubt there to provide additional manpower. This is also highly useful for the Golden Company – while they’re some of the best soldiers in the world, there are still only 10,000 of them and if you’re going to go up against the Iron Throne, you need manpower.

Alequo Adarys provided finance and a credible puppet-ruler in Tyrosh, and lasted the longest out of all of them. 

So yeah, I think the Band of Nine were overall a positive. The only drawback I see is that trying to grab Tyrosh first might have delayed an invasion of Westeros, but it’s not like going straight for Westeros had been particularly successful in the past.