The dealer will typically take the attitude that the service had not been done, and that is why they are insisting that you take the bike in for service. Now, if the previous owner of your bike did his own service work, he MUST have records to prove the work that was done, and the record should have been passed on to you. If not, then potential warranty work will be jeopardized, and to fix that you should take the bike into the dealer!
Just note though that there is absolutely nothing wrong with doing your own maintenance work, and BMW canNOT use that to void any warranty work. Something like that had gone to court a couple of years back, for an owner of a Mini car, and BMW had lost. I do all of my own maintenance, except for the 600 miles (to give the dealer a chance to go over the bike thoroughly after their initial set up when new), but it is important to keep meticulous records. I use BMW's own maintenance check list (they can be downloaded) to check off all items that I did, then dated, record the odometer reading, and sign as if the dealer had done the work. I also attach all receipts of the oil, filter, etc. to show exactly what oil, what filter, and so on, was used for the service.