I'd agree that theory grades are worth doing, they'll vastly improve your understanding of harmony, structure, melody, etc etc. In a popular beat combo context, its unlikely that the fact you have passed an exam is going to make you more attractive to potential bandmates. I've got ABRSM grade 8 theory and it taught me a lot but impresses nobody.
Practical exams are worth doing also, but I'm not familiar with the Rockschool type ones, just the classical ones.
Do it if you want to increase your knowledge and ability, not to add to your CV.