I'm pretty sure there are multiple reasons for the 13" not being a quad core machine.
1. Price
2. Quad core draws more power and the smaller chasis means smaller battery size.
As for its speed. For 90% of people the Dual Core i5's and i7's are going to be more than enough.
I have a 2011 13" MacBook Pro with a 2.3Ghz Dual core Core i5 CPU and the CPU in the early 2011 13" benchmarks higher than then entire line of 15" and 17" MacBook pros from 2010 which is the first time ever for a Macbook pro.
The core i5 chips really made the 13" a power house as previously they only put slightly faster Core 2 duos in each new model every year and performance gains were minimal but with the Core i5 it doubled the previous year in speed.
As I said, i have a 2011 MBP 13" with 16GB of Ram and a 256GB SSD and their is nothing it can't do besides intensive games and thats just because of the GPU.