Except that the i5 is not a low power one, performs alot better. The GHz is also higher on the MBP so I fail to see what you mean by that?
Consumers don't look at the TDP. It's not something Apple advertises. Apple doesn't even tell you what generation of Core i5 or i7 is in their MacBook unless you dig.
If the marketing sheet looks like:
- Air: dual-core 1.8 GHz i7
- MBP: dual-core 2.3 GHz i5
The 1.8 GHz i7 would compete with the 2.3 GHz i5. Consumers will think the i7 approaches the performance of the i5. Everyone is trained to look at GHz and Apple lists that as their first spec.