Buying a Tesla with bitcoin will most likely be a great sales gimmick, to generate attention around Tesla.
May be they wanted to create a hype and a selling point to attract wealthy investors but from what i understand they are having good sales even without much advertisement.
But I think only a few people will use the paying methods, as most people who hold enough bitcoins to buy a Tesla, understand that the real value of bitcoins is much higher than the dollar price bitcoins has right now.
If you are having monetary restrictions then you are not going to use them to purchase anything in life. The investors who are able to buy Tesla using bitcoin are not going to be small time investors and hence they do not compare them with the future valuation if they wanted a car now.