Time of the Doctor: Tasha Lem? Why?

Or to be more accurate, why was this "mature woman" with a perverse interest in seeing the Doctor naked not River Song? The answer is because Moffat needed to pad the plot out for another two minutes.
Let's recap: Tasha's set up a church that you have to be naked to go to, but you're allowed to wear a hologram in order to keep the censors happy. The entire sodding point of this is to do a gag where the Doctor shows up to Rose's Clara's family's Christmas dinner in the nuddy.  (And then, apparently, so Jenna Coleman doesn't have to wear a bra for one day of shooting - no, don't look at me like that, I read it on TvTropes.) I could grouse about having Clara's family in it at all, because until Arthur Weasely showed up in Series 7a I'd assumed that companions' families were a relic of the RTD years. But as I mentioned in my first impressions of the episode, there was a godawful amount of marking time in it, time that could have been better spent wearing down the Doctor's spirit. 

Seriously, here is the plot of The Time of the Doctor in a nutshell: the Doctor lands on Trenzalore and finds that there's something so important there he can't leave until all his enemies are driven away. He can't drive away the Daleks because they're too powerful, so he stays pat for 300 years (somehow aging dramatically being forced to wear layers of latex despite the fact that the first 400 years of this incarnation didn't add a day to his baby-face). Finally the Daleks do the obvious thing and start leveling the town, but the Doctor is granted a new set of regenerations at the eleventh hour (see what I did there) and uses the first one to defeat them. Everything else - the Doctor sending Clara away twice, Clara's family, the sodding naked church - is padding.

So why Tasha Lem and the Silence? Okay, I actually liked the idea that the Silence we met in Series 6 were a renegade splinter faction and that most of the Silence are actually okay (if kinda creepy) guys. The notion that any race (except the Daleks, Cybermen, and Sontarans, which are explicitly programmed/bred for it) could be Always Chaotic Evil is rather irksome. But let's take a look at Tasha:

She's an older woman who gets to do lots of flirting with the Doctor. She's got an explicit connection to the Doctor and the Silence, and although her motives are ambiguous to begin with, she is definitely on his side. Moffat could have slotted River Song in here pretty easily. After all, the easiest way to show that most Silence are good guys would be to have them reach out to the woman their renegade cousins royally screwed up. Having these good Silence lurking around River Song the same way as in the final product would add more tension, because even though we think we know River by this point, the Silence have this rather important connection to her. But, of course, there's one problem with this plan, and it's not Alex Kingston's availability: it's that Tasha Lem dies, only to be resurrected as a Dalek infiltrator, only to shrug off that programming in the span of about two minutes, and the whole thing is never brought up again. I know hideous padding when I see it.

Y'know, you really diminish the power of an enemy (and thereby also diminish the dramatic tension) when you show that every important character can resist them. Hey, remember Planet of the Spiders or Pyramids of Mars? Where the Doctor gets possessed and is utterly powerless to stop it? Yeah, now we have "Victory of the Daleks," where concentrating enough on fake memories is enough to make you human, "Asylum of the Daleks" where Dalek!Oswin can resist her Dalek programming because she's a pseudo-companion, and The Time of the Doctor, where Tasha Lem shrugs off her Dalek programming because, hey, it served its purpose of throwing us another pointless plot twist and padding things out for another two minutes.