An "all-in" quote has the advantage of certainty; But a solicitor who operates this business model will price accordingly - i.e. the solicitor in setting a price will assume that outlays are in the mid-range. In some cases the outlays will be more, in some less; The solicitor will on average do ok: some clients will lose out and some will profit from this averaging.
The 'all-in' quote has its place, and the certainty which it brings is itself of value to some consumers; but I am not persuaded that on average it delivers better value.