"The arguments in favour talk about efficiencies of scale leading to cheaper and better services..."
Let's take these two claims and examine them, eh?
1. [Efficiencies of scale] - These two companies compete for the same market share and have fully redundant operations. On face value, it appears that this claim works, if the majority of one carriers staff gets sacked and the remainder work harder for the same money. However, in this economy, approving the sacking of the majority of T-Mobile (you didn't think that AT&T was the redundant player in this equation did you?) employees appears badly short sighted (I suspect that is the reason for the delay -- can't have massive layoffs during an election year). So call this one TRUE, but also A BAD DEAL for everyone but the top brass and investors.
2. [Cheaper and Better services] - okay, 2 ways to look at this. Historically, reduction in competition always leads to higher prices and less innovation. So historically, this is not true.
If either of these companies currently had plans or was able to offer cheaper and better services, they would do so in a heart beat to gain market share and reap the benefits. So, logically, this is not true. I would also stipulate that in this modern era, anything that reduces costs is absorbed by bonuses and investor payouts, never to be seen by consumers (seriously, when did the North American consumer in Telecommunications last see a true rate cut?)
I also think the spectrum acquisition for improved service has been hung, drawn and quartered.
If the FCC votes to allow this, the public will once again be taking our turn in the Pillory of Greed. I can almost feel the campaign contributions flowing as we speak.