It seems your memory is poor. The issue was that both drivers received a new front wing. Vettels failed, so they took Webbers and bolted it on to Vettels car. Webber vented his fury in the press conference after qualifying, slamming his glass down on the table after saying "I think the team is happy with the result". Webber wen't on to win the race and famously said over the radio "Not bad for a number 2 driver".
I agree, most teams do prefer one driver and that driver can sometimes get development parts before the other. Hamilton Vs. Kovalainen is hardly a good example now is it? Hamilton won the championship in the 2008 McLaren, where exactly was Kovalainen? You could argue Kovalainen was given poor equipment, but he doesn't exactly have a good track record.
Trust me, I will be the first to scream and shout the day Mclaren give development parts to Hamilton, and not to Button
"Unbelievable how silly this Formula 1 is these days, with this stupid overtakes."
—Sebastian Vettel, 2012 US GP
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