The 28mm is a great length and I had one for years but I'm trying to go lighter yet while still covering as much "ground" as I can. So I go with the 25mm and 35mm. I wouldn't give up the 35mm since it's my fav length... and I need the 25mm for those very wide shots, so it makes the 28mm the odd man out.
Now for those who shoot with a 50mm, then I would go with the 28mm and the 21mm. It almost depends how wide you need to go. These would be my two systems.
25, 35, 85
or
21, 28, 50, 85
-rob