1) The same argument was made before dual core made it to consumer PCs. If you build it, they will come.
In any case, waiting on either PC or phone is usually due to some IO task, not heavy CPU usage. By far, the most waiting I'm going to be doing is when web pages are being loaded.
Media playback and games are primarily where users will see the most benefit from dual-core in the foreseeable future. Having a heavy webpage with Flash running smoothly doesn't hurt either. :)
2) Today, chips have very good power-gating. If only one core is being used, only one core is being powered. Also, the power usage increase is logarithmic. For this reason, having a second core doesn't double the TDP of the entire chip.
Also, most of these dual-core chips add a fraction of die space in return for an extra core. The SOCs already only dedicate a minority of space to the ARM core- the rest is taken up by the GPU, Memory, radio, and other misc controllers.
And due to die shrinkages with every generation, many dual-core chips will be drawing less power than their single-core counterparts. Case in point: the 3rd generation Snapdragon with dual-Scorpion cores is claimed (at least by Qualcomm) to use less power than the Snapdragons in current smartphones. Going from 65nm to 45nm (28nm expected by end of 2011!) provides that kind of headroom.
Besides, the biggest user of battery space is usually the screen, then radio (wifi, 3G/4G, bluetooth, etc), then the CPU at a distant third.
Double core- Double battery usage? Right, whatever.