Trying to get someone in to repair our boiler mid winter proved impossible which left BG as our last resort a few years ago. In all fairness they had someone out the same day once we'd signed up and paid them a one off £100.However, in our case, this proved to be the start of a total of 18 visits in a two month period. The cause of these was three fold. We rarely had the same 'engineer' twice, the degree of knowledge varied significantly between each engineer,and each visit would need to be followed by a further visit 3 days later while they sourced the part.
They did eventually fix the problem, but only once they'd thrown just about every conceivable part at the boiler. (A good thing long term possibly)
Some of the engineers seemed to have some inkling as to what they were dealing with, but most relied solely on a series of flowcharts on their laptop. I would imagine its pot luck across the country in terms of whether you get a competent engineer or not.
Is it worth it? You could argue the same for any insurance you shell out hundreds of pounds for each year and never claim.
If you can find yourself a reliable and competent local engineer, then I would put the money aside each month and treat yourself at the end of the year if you don't need to use it.