In the 'bad old days', précis was a major and compulsory part of all O Level English Language examinations, accounting for about 20% of the overall mark. During the 1970s précis ceased to be a mandatory O Level exercise, a change upheld with the advent of GCSE. No English GCSE syllabus that I know of insists on précis competence; several do not even make any reference to it.
I think this is a shame. On the whole, those latterday changes in O Level were for the best, and GCSE has proved an exciting and imaginative replacement, allowing pupils and their teachers much greater scope than before. Far from slipping, standards have in my experience risen, and there is little doubt that English work at 16+ level is much more enjoyable now, both to teach and to learn. But the ability to précis is important; some would say it is the central language skill. For a start, it is a craft essential in all professions and businesses; indeed, anyone whose work includes dealing with documents at some time (and that accounts for most people) will need précis skills as a matter of course, or live to regret their absence.
Such vocational considerations, though important, are not in my view the most telling, however. The fundamental value of précis is that it tests and exercises every aspect of linguistic competence. To write an accomplished précis you need to have mastered eight major skills.
1. Good comprehension
To reduce a long document to its essentials requires a sound understanding of its every point and sentence. This is often more demanding [and therefore more reliable as a gauge] than any standard comprehension exercise.
2. Good prose composition
A précis should be crisp and easy to read. To achieve such at-a-glance clarity you need a muscular style, where everything is pertinent and works efficiently.