We just got back from our honeymoon in Italy, and we used an agency that only does Italy trips. We went to 7 different cities, and our agent was able to book our hotels, car service, train tickets, and any excursions we wanted ahead of time. We added some things in while we were there on our own, but also had our agent available to suggest a replacement when our boat tour got cancelled. As it was our first time there, it was nice to not have to worry about setting all those things up / figure out the timings needed.
Would I use an agent for every trip? Absolutely not, but certainly for specific trips for sure.