I would not for a couple of reasons.
1. Reception. Biggest downfall is the degradation of wireless signal, for cellular, WiFi, bluetooth, etc. A big slab of aluminum on the back will hurt my already borderline reception, due to it being a big Faraday cage.
2. Weight. I know it is very minor, but aluminum is about 3 times denser than ABS/PC (what the plastic back is made out of). This also assumes that you are using aluminum pieces that are as thin as the current plastic pieces (~.050").
3. Ease of disassembly. The back plate is hard to get off of now, and it is flexible. An aluminum part might be hard to remove, depending on design.
4. I don't really need an increase in strength or protection as I use the Verizon silicone cover. I've dropped it numerous times with nary a scratch.
As far as heat, it will conduct heat greater, but won't be a great heat sink unless the outside is ribbed, or open to a high convection environment (i.e. not in your pocket or hand). Also, it won't do you any good if you have it in a case, like me. But I think this is aimed at people without cases.