I received a letter from MVG a couple of weeks ago, and it looked like a generic "Welcome to Munich" packet (my German is rubbish, so I've no idea what it actuall said).

Today they've sent me another letter talking about a Servicekarte. Is this something I should actually pay attention to? Why are they sending me letters?

Its a welcome to munich pack.. Pay attention to it if your intrested in the services the companies offer. If you not then chuck it..

Thank you!

After the second letter I was afraid I might have to register with them to be a resident user of the bus, or to fill in an interminable form for the entertainment of the drivers, or something. Amazing how quickly paranoia sets in after a couple of weeks of filling in forms for every conceivable eventuality.

You register yourself with the KVR. MVG is the local transport company, why would you register with them
unless you looking to get a monthly/yearly transport card..

I don't know. The same reason I have to fill in a form to request a form for reimbursement of expenses. Or the same reason they gave for refusing to reimburse me for a Ryanair flight because all I had was the boarding pass and the itinerary, not the paper ticket: "That may be how it works in other countries, but this is Bavaria".