It depends on the cruise lines. Some of the lines now are not allowing discounting, in theory. If they find that an agent is discounting then (in theory) they'll suspend bookings from the agent. OTOH there are other things that can be done other than a lower price to sweeten the deal, like onboard credits etc.
One thing to remember is that most agents don't get 100% of the commission themselves, many only get 50%. The agency I used to work at specified that if I wanted to discount that was fine, but it came out of my half of the commission. Commission is based on the actual fare only (not including port charges, government taxes & fees and the "non-commissionable" fees), so if the actual fare was $1000, then the total commission would be $100, out of that the TA would get $50. That doesn't leave a whole lot of room for discounting, and saying "I can knock $25 off your fare" doesn't sound like a whole lot.
Out of the remaining commission that the TA gets they still have to pay for long distance, related courier fees (if there are any) and any extra perks they decide to give you.
So some folks might get some cabin perks (like shipboard credits, gift baskets, special surprises etc) and some might not, but might have gotten a discounted fare. Sometimes it's not possible to do both, depending on how the finances work out.