I am required to do the same, Juan.
I think this is intended to give musical shape to the scales, and also to practise control.
Another reason I was given was that the high notes towards the treble end are softer and "thinner", hence if you play with no dynamics, the scale will eventually sound softer on the treble end.
I know that many forumites are practising cresc/desc on their scales, so I am quite sure if it is done well enough, the examiner has no reason to view it as irregularity.
On page 38 of "These Music Exams", there is such a bullet:
Candidates will also be assessed on their abilities:
- to perform the prescribed technical exercises for the grade (e.g.
scales and arpeggios) with fluency, accuracy, evenness and musical
shape
Similarly, on page 41, there is such a marking scheme for scales and arpeggios (19-21 marks):
- Quick response
- Fluent and musically presented
- Confident and even tempo
Both pages talk about scales being "musical", so probably some dynamics will be really helpful in conveying your intention of making scales more than just a boring technical exercise.
LINK:-
These Music Exams:
http://www.abrsm.org/resources/theseMusicExams0607.pdf