It makes a lot more sense to evaluate an ISP based on the speed and latency they give you for the price (along with other factors like service, reliability, ethics, etc.), not how much of your modem's theoretical maximum bandwidth they're able to use.

In order to be DOCSIS 3.0 certified, your modem does have to support 4x4 bonded channels (4 channels up, 4 channels down). But that does not mean your provider has the channels available or will provision them for you. DOCSIS 3.0 modems will happily provision themselves down to 1 6.5Mhz channel down, yielding just 38Mbits as a maximum.

There's a lot more to it than just channel allocation, as well - the 38Mbit maximum for a 6.5Mhz channel is with 256QAM and a good signal-to-noise ratio, which depends on a huge variety of factors.

Plus, even if you're getting plenty of channels allocated and have a good SNR, there's still the backhaul from your nearest point of presence into Comcast's WAN, the bandwidth across Comcast's WAN, and then whatever peering agreements they have to get across to the Internet at large.

At any rate, I think this is a pretty silly argument against Comcast - it's like being angry that your Gigabit Ethernet switch doesn't give you 1000MBit access to some random server on the Internet.