Should you use a travel advisor/agent?

The quickest answer is sometimes you can/should use a travel agent/advisor. They are now calling themselves advisors (a recent change).
All I can do is tell you when I use a travel advisor.
1. I use a travel advisor when I am booking cruises.  Often they offer additional credits beyond what the cruise line is offering. For instance on my recent booking to the Baltic CruCon (https://www.crucon.com)  www.crucon.com, offered a $150 USD on board credit. They are a high volume cruise booker so it is likely based on volume??? Not sure.
2.  I use a travel advisor at Expedia Cruise Centers when I have found the best deal, and its thru the Cruise line only. I don't pay anything extra, she gets the commission, I get expedia points that I use for hotel bookings thru Expedia and I have the added protection of going thru a travel agency.

When do a book on my own?
1. I do it when I book flights and hotels only.
I often find that I like the convenience and safety of booking the airlines directly on their site. The price (even with sales) are the same and I can book my seats and pay for my bag all in one transaction. They are the best at keeping me informed on any flight changes as I have found Expedia has had an issue with the flight changes as they always ask me to accept the change (I do) and then for some reason it doesn't take.  I caused me a whole lot of hassle with a Delta/WestJet flight last summer and the Expedia supervisor was extremely rude and still didn't fix it so I had problems checking in with WestJet. Thankfully West Jet did the best they could and I got on all my flights, but had to check in with agents versus online and kiosks even though I only had a carryon.

With Hotels I use Expedia and/or Booking.com.  I use Expedia, (Expedia.ca) when I am using up the credits I have earned from Cruises and hotel bookings.  But I like Booking.com  (booking.com) the best as I have the Genius level discounts from booking many hotels and I trust the ratings of people who have stayed at the hotel.  I find if the place has an average guest score of 8 or more out of 10, regardless of the hotel star rating ( I usually stay at 3 star) I can trust the experience I will have.

2. Booking trains and buying passes in Europe.  I find booking directly with the national train service or tourism site for city passes the best way to go, as you can trust the reservation, don't have to pay a fee and usually the price is the same.  However I have had an issue with Canadian Credit cards with the French rail service - for some reason they didn't like my card.  So I had to book with a third party and the ticket was fine but there was a small fee.

Hope this Helps
Marcia