From a marketing standpoint (as well as R&D), this is rational. Common marketing mechanisms for software like this are very segregated -- Hardly any conference interesting to people using Open Source stacks will have much Microsoft content, and vice versa. Blogs, magazines, etc. are all similar in this regard. So, the marginal cost of marketing another developer utility for the MS platform will be much lower than the cost of marketing the first, say, DB2 utility acquired.