I am a Disney Destination Specialist (TA) so I'm a bit biased but as the PP mentioned you can't lose by using a TA. If they are good they will help with ADRs, itinerary planning, keep an eye on discounts, book any tours, etc....they take all of the busy work out of your trip.
Make sure they don't charge you for their services ~ they should be complimentary (Disney pays us for bookings, our concierge services are free).
Also, try to find an agent/agency that is an Authorized Disney Vacation Planner. This means they have met certain criteria with Disney. And the agent should have attended the Disney College of Knowledge.
Using a TA should in no way affect your odds of getting a PIN (make sure you are registered on disney.com as I believe that is how PINs are triggered). We book many clients who receive PINs.
Happy Travels!
