I kept it throughout the Julii campaign, but mostly because it helped me take Carthage and what not. I don't remember it ever making me much money, but it did turn a profit.... in the long run, you'll make money off of it, even if it is producing around 250 denarii a turn...you're still making money. It just depends on whether or not you need the cash right away.

I do recall that in MTW I used to drop provinces that cost too much to hold and produced too little income, but in RTW with the garrison costs spread across several cities, it would just offset the cost to another city.