One of the concepts about levying a gas tax is that it will force people to drive less. If people drive less, the it means cleaner air. Perhaps it'll force even better fuel standards, and better for our national security.
According to this decently balanced post from Money, there are valid reasons for allowing the gas tax to expire and also good ones for keep it in place or even increasing it.
By allowing the gas tax to expire, it would mean that the states will have to find a way to maintain their parts of the road. That would mean they'll be the ones having to increase the taxes on their own (GOP controlled state legislatures and governors may not like that). It could also mean less red tape and perhaps in some states, unless involvement of the unions.
And it could be good for the states that are not part of the national highway system or has much involvement, they no longer have to pay for everyone else's roads in other states.
However, that would quickly fall into regionalism. Being that we are still one nation, it could be unfair to many states in the middle of the country. Hence, the businesses advocate an extension of the federal gas taxes. And in a few cases, some even advocate an increase. GM is one of them. On top of that, as fuel efficiency increases, people are paying less gas per mile driven, which means less tax collected.
I walk to work and drive on weekends if I have to. So a higher taxes that will keep the rest of you off the road is fine by me. Maybe a higher tax rate on gas at the federal level could have us seeing fleets of airships delivering cargos instead of smelly and air-polluting 18-wheelers.