Firstly, the poster says 49 (the website says +vat) so you need to check
If your boiler is in good condition and regularly serviced, breakdowns SHOULD be minimal
so overall it doesn't benefit IMHO, if it did why would the service be offered?
Most companies who offer this service are very particular what boilers are allowed into the scheme (for obvious reasons) and look very carefully at what is actually covered, for instance I have seen people call out a company with this type of cover only to find that the particular breakdown wasn't covered, like a burst heatexchanger or something
Suppose what I am trying to convey, if it's a little too good to be true.......isn't there a saying?