In general I prefer to wear clothing made of natural materials. Cotton, wool, silk, etc.
But I have to disagree with what has been written above about fleece and wool.
Fleece is not heavy. Synthetic materials, like fleece, are about twice as light as wool for the same insulation properties.
The only material that beats synthetic is down, but it is useless in wet conditions.
It is true that fleece stinks easily, but it is also very easy to wash and dries really quickly.
Merino wool is indeed a fantastic material, but its properties are sometimes exaggerated.
Many mountaineering and polar expeditions have failed because of woolen clothes freezing.
In answer to the initial post. I do wear bike pants on all my trips. Only in Iran and Pakistan I put some long pants over them in the cities. Almost everywhere in the world cyclists use cycling clothes, like soccer players use shorts and soccer shoes everywhere. The first days in Iran I cycled in long pants. But some local road bikers, of course in bike clothes, told me it is silly in this heat. I agree with Monkey, the bike shorts work.
I usually wear an old long-sleeved shirt, to protect the arms from the enormous amount of sun you receive on a bike tour.
A friend of mine used the Shimano sandals with SPD. She really liked them. And she looked comfortable in them. I think she took the clips out of the shoes when she was about to walk a lot. It is easy with an Alan key. One think I noticed is that her feet got really sun burned, with the funny pattern of the sandals and her feet were always very dirty from the dirt that comes of the road. Minor problems, but something you can keep in mind.
I take the following cloth on a long bike tour:
1 cycling shorts
1 long sleeved shirt to cycle
2 pair of socks, one for cycling, one warm
1 pair of cycling gloves
1 cap with flaps over ears and neck
1 thermal shirt
1 zip off pants
2 100g fleece jumper/vest
1 wind and water proof jacket
1 pair of good water and wind proof gloves
1 pair of water proof pants
1 warm hat
1 pair of cycling shoes
1 pair of very light sandals
1 pair of booties for over my cycling shoes
This equipment took me across deserts, tropical rain forests, Tibet and the Andes with temperatures ranging from -20 to +50 degrees.
If you don't expect extremely cold weather you can leave one fleece jumper and the booties at home.