I get asked this all the time by customer, "is it really worth having my boiler serviced?".
If your boiler is under a manufacturer warranty, to maintain the warranty then it must be serviced annual. Failure to do so will void the warranty, so if you have a new boiler with a 10 year warranty but neglect to have it serviced after the 1st year, there was no point in paying the extra for the boiler with the longer warranty.
Once that warranty has expired, the benefits of having your boiler serviced are;
Increased efficiency - Removing deposited from the heat exchanger will maintain the heat output of the boiler.
Increased reliability of boiler - having the boiler inspected every year can highlight leaks for example. Before they causing any damage to wiring and circuitry.
Reduced chance of Carbon Monoxide Poisoning - Having the boiler seals inspected every year will highlight any damage and they can be replaced before any fumes leak.
Convenient Servicing - Planning your service can be done at a time that suits you, rather than waiting for the boiler to breakdown. Forcing you to take time out to get it fixed, which could have been avoided.
So yes, I believe that have an annual boiler service carried out is worth the relatively small cost.