Thomas - you have a PM
Although a handling agent isn't 'mandatory', parking space is extremely limited as has been said before. Now, for a quick stop (up to 30 minutes) airfield ops will park you near the terminal for a very reasonable price which you can pay direct to the airport.
Any longer, and you'll be directed up to the North end of the parking area where you will be forced to use Signature, as you'll only be able to access your aircraft via them. And once there you are a captive audience - you become one of their customers.
No other way round it I'm afraid