Hi,
SAP does not to restrict us with one module learning, only the domain exp will play a major role when we discuss the SAP aspects with customer and to map what customer wants.
As a consultant, we are intended to Map the customer requirement and business process.
To your case, MM is an essential module compared to PM, since most of the implementations will have MM, but PM is not such mandatory. I have seen projects where PM is handled by PS/MM/PP consultants 
I am a Certified PM consultant, experienced and worked with PS module so finally working on both PM/PS now. If you dont want to lose MM, then you need to understand the process what they use in MM and learn all MM aspects including PM. When you complete 3 years, you should be in a position to handle MM also.
Regards
Terence