The TB connection is for scavenging blowby when the intake is below ambient pressure, ie; no boost. Under those conditions the accordian line supplies fresh air for dilution.
Under boost the flow reverses and all blowby is sent to the accordian. This boost "leak" also helps to scavenge the valve covers by venturi effect. Think about blowing across the top of a bottle.
Since the PCV line going to the accoridan is upstream of the turbo there isn't enough negative pressure there when below boost to scavenge properly. Capping the TB port may not cause any apparent problems but it'll reduce scavenging when not boosting, which is how the engine runs 99% of the time (well, for most of you guys
). The result will be more sludging and acid build up in the oil, especially if blowby is excessive.
Put simply, the TB port handles blowby when not boosted and the accordian line when boosted. This is why the turbo cars have the line and N/As don't. Forcing the accordian line to handle scavenging when not under boost doesn't work well but people do it anyway.