Very right sir, It's better to have something than nothing - I will say that make it mandatory for voter as well and bar people from cast vote if they don't file tax return. But - what i see is that it will not make dent on PPP & PMLN as you are perceiving because they have money to buy FBR official and getting Tax Return by bribing is not that much difficult, many people who apply for immigration do this - so for status quo mafia it's not a problem, but for people who honestly paid their taxes but didn't filed their return because of laziness are gonna lose.
Anyway - Let's see, how ECP deal with this.