That's got to be the reason, and really, it makes a lot of sense - a fair approximation of Ukrainian's gotta be hard.
Helena's my favourite clone by far, and while I understand that they've got to whip through a lot of plot to finish the story in the episodes that are remaining, series-wise I wanted more backstory than that and for her not to have been so spectacularly sidelined this season, but oh well, I'm still enjoying it, I love the whole series and I'm still looking forward to the last episode.
@WatchrTina - for me, there's been quite a few times this season where I'm seeing either TM or Sarah in the other clones. I think I can fanwank that they've been the moments when Allison, Rachel and Helena have looked so beat down and tired, or like you said vulnerable, because I haven't felt like that previous seasons... but not sure.