I think a portion of their argument is that it allows smaller ISPs to gain footholds in niche markets. By allowing them to prioritize and shape traffic, they can gain advantages and offer services that the larger telcos won't.
I imagine an ISP that focuses on serving low-latency gaming traffic at the expense of other application's bandwidth. Or perhaps an ISP that rents bandwidth from a larger one and provides its customers with better bandwidth guarantees on weekends and nights (or balanced out with better business-day bandwidth for commercial customers). And in some ways it makes sense from a market perspective (that's generally how smaller companies can innovate and create advantages).
What all that ignores is the immense locality of ISPs in general. Unless things fundamentally change in regards to how the internet is delivered in the US, most people are never going to have meaningful consumer choice in the one place they live. Giving ISPs more control over bandwidth just gives them more control over consumers.
It's like gerrymandering. The most negative consequences aren't that it gives partisan advantage, it's that it gives representatives the power to choose their constituents.