As for why some wikis don't have adverts, I would imagine they are only monetised when the wiki reaches a sufficient level of activity/size. My test wiki is lacking adverts because of this.
Yes, but this announcement occupying a part of the mainpage. In other wikis I have this ad does not appear in the same place. This very disrupting the mainpage design. It's a shame! At least at main page I wanted a larger space for the layout.
See my wikis and the difference between them. An advertisement appears in no other.