I'd suggest that Microsoft appears to be doing the opposite lately, as evidenced by Acompli and likely Sunrise. But I agree this is generally the case.
Agreed, Microsoft's approach with Acompli has been different, but that's also a fairly unique situation where Acompli only existed because of a lack of a dedicated client targeted at Exchange customers - it may be that the purchase was as much about appearances (i.e. not reliant on a 3rd party for mobile) as anything else.