Let me clear the general misconception that the level of MSc Maths at BITS Pilani is similar to the 10+2 level. Class 12th Maths is highly analytical while MSc Maths is full of definitions, theories, and proofs. The pedagogy adopted is poor and the majority of students study this subject despite a lack of interest.
For someone who has no interest in Analytical stuff, courses such as ERA, Topology, and Functional Analysis can be very challenging. Some courses which will intrigue you will be of no use because BITS follows the relative grading system where you are graded according to how well you perform as compared to others.
Overall, if you like complex theories and expressions then go for it, but if you too have no interest in complex theories it’s better to not take it.
While many claims that Maths+CS is the best combo, let me tell you that you can take 3–4 Maths courses that help in CS as electives in your CS degree. MSc in Maths is a suitable option only if you are willing to pursue a career in Mathematical research, else the best dual degree option is MSc in Economics. The curriculum is light and knowledgeable.