I love 28 but as Roger said, it's not wide enough for real wide vistas or some architecture stuff. For me it's simply the shortest I'm comfortable using as a walk-around lens, the most dramatic and involved "normal" lens. Anything wider is for specific needs. As someone as said earlier, it's better to concentrate on the shots you can get with the lens or lenses you have with you than trying to cover everything. So I make do without a super-wide because that to me is only useful for a few very specific shots, too much of it quickly gets, well, too much. But if these shots are important to you, by all means get the 21, it's a wonderful FL and the G 21 is supposed to be a wonderful lens. Pair it with the 45 if you use the 21 less or the 90 if you want to use the 21 for more general stuff, better don't take 45 and 90 both - at least for me, three so widely spaced focal lengths would be too difficult to wrap my head around and would precisely put me into the mindset of trying to get every conceivable picture rather than concentrating on what's in front of me.
Morocco can be tiring to travel in because of all the people trying to sell things or services, but stay relaxed (take breaks!) and friendly (you'll need to say no thanks - la shukran - very firmly, often three times, to get them to stop, but if you just say no without thanks, they can be seriously offended! Not incidentally, just saying you're not interested like you mean it is also the most effective way to haggle prices down) and you'll also experience immense friendliness and hospitality.
