To me, I prefer to watch outdoors matches, particularly on natural surfaces. I just think with indoors there's really not many added elements that players have to battle with. I think the ATP sees things this way as well, hence no indoor slams.
You could say that AO and WIM have some matches indoors, but that's mainly to avoid the tournament stretching out because of rain. In their purest form, they are intended to be played outdoors.
As for Nadal winning his first indoor tournament in years, how many has he entered in the past 8 years?