No one is proposing the gas tax be eliminated, or promising that it won't be raised in the future. Asking for the 'defect' in the current system is like asking the question of why we need sales taxes, property taxes, inheritance taxes, gas taxes, tariffs, telecom taxes, corporate taxes, sin taxes, license fees, alternative minimum taxes, and capital gains taxes when we already have personal income taxes.
The fact is that we never need another tax; we already have plenty enough. But the more places that you insert the taxman, the more total revenue you can extract without people noticing and raising a fuss (cf. VAT).
"L'art de l'imposition consiste à plumer l'oie pour obtenir le plus possible de plumes avec le moins possible de cris."
("The art of taxation consists in so plucking the goose as to obtain the largest amount of feathers with the least possible amount of hissing.")
Jean-Baptiste Colbert