I can't say this is going to make a big difference in how I use pandas but I've ran into the bizarre "can't have nans in an int Series" annoyance in almost every pandas project I've worked on, so good on them for fixing that.
For a more technical discussion of the performance implications of one possible implementation, see http://wesmckinney.com/blog/bitmaps-vs-sentinel-values/
Not allowing nullable type in raw integer `Series` is the least bad solution this problem. If you really want nulls in numerical data, either use floating point or `IntegerArray`.