Without considering the subjective question of whether MidpointRounding.ToEven (Banker's rounding) or MidpointRounding.AwayFromZero would be a better default.
When designing this, Microsoft would have considered the languages .NET was designed to replace.
VB classic has always used Banker's Rounding by default.
C / C++ conversions truncate when casting, and has library functions floor() and ceil() in the runtime library - but (AFAIK, may be wrong) no round function.
Java has a Math.round that in the documentation is described as being equivalent to Math.round(a+0.5). Which is arguably not what most people would expect for negative numbers (-3.5 rounds to -3).
VB developers may arguably be expected to need more handholding than developers coming from C/C++ or Java.
Hence it seems reasonable that when designing .NET, the class library would provide Floor, Ceiling and Round methods, and that the Round behavior would default to VB's behavior.
It also seems reasonable that Convert.ToInt32() would use the Round method (though I guess a case could be made for Floor, for consistency with casting).
