This has been an interesting thread. Everything from spy quests and roaming buildings to the actual topic.
1. I support the idea of multiple types of spy build (attackers, balanced and defenders).
2. I do not support the idea of giving buildings bonuses such as spies with the building. However, w33lz idea of regen, troop count, plunder, and so on bonuses are good.
3. The easiest to way to encourage switching is to alter the plunder bonus given to each type of building. While I saw that the devs mentioned they were disinterested in this idea I think it would sustain the longest amount of switches.
4. I think the devs may be jumping the gun on these changes. While I like that they are being reactive/proactive about wanting to balance the builds I think as clans begin experimenting with builds inside the new war system more mixed builds will appear.
5. Another benefit which should be examind is troop loss in accordance with defensive buildings. A def building should be better prepared to withhold a siege than an attacking building. Thus they should lose less troops when they defend successfully than an attacking building loses.