when they say wireless N has a throughput of 300Mbps, it is never this is it, isnt it 46Mbps using 2.4Ghz and 97Mbps using 5Ghz?
You can get 300Mbps on 802.11n 2.4GHz if you use 40MHz channels, but then you can only use one channel. Unless you are in an isolated area with no overlap or interference, you won't get anywhere near 300Mbps, usually < 144Mbps. 5GHz gets you up to 450Mbps, or using AP's and clients that support bonding 2.4 and 5GHz channels to get you up to 750Mbps.
As to the original question. 2.4GHz is useful as it's range far exceeds that of 5GHz, so in low-density areas without much traffic or interfering AP's, it's preferred. 5GHz is much more ideal for congested and high-density areas as you can have more AP's in a given area using many more available channels, plus 802.11ac draft 1 supports speeds up to 1.3Gbps.
The good news is that the FCC just approved an additional 100MHz of spectrum below the current 5GHz WiFi spectrum for added capacity and more channels. However there is push from many to allow WiFi standards on the lower frequency bands such as 600MHz.