Thing is, what risk(s) are you insuring against. If I take my bass to a gig the most likely event will be damage of some kind and from some source. OK, I know of a bass that had an upright piano fall on it, so a total write off. But £2,000 will easily cover quite a serious amount of damage. Now, digging deeper on this, to a student for example a DB may well be his principal asset and he would do well to cover his instrument for total loss, eg theft, or in a house fire. My house is equal to an awful lot of basses so it makes sense to get that well-covered and in the event of that getting destroyed I might be less concerned at losing a bass.
We can go through life insuring everything against every risk - and spend an awful lot of money doing so. Or we could cover ourselves against the biggest risks and take the lesser events on the chin.
(Commenting on Rabbie's post - quote missed off, sorry)