^ If that were true, then all the people who did NOT use Sylvia-related additional texts would have been stuffed. As it happens, I wasn't (I used To Kill a Mockingbird, and still got a Band 6 for adv eng), so that disproves that theory.
The reason I'm finicky about having people chose Sylvia's poems are that a) almost everyone in the state will do them, and b) way too many people fall into the trap of focusing on their relationship. And considering that the vast majority of people tend to side with either Hughes or Plath (as lizabella has stated above), they don't analyse the texts from the perspective the syllabus wants.
From an essay standpoint - YES, if you can make your focus how truth is represented in the text, using representations of their relationship as one of your arguments, fine. But remember that the prescribed text is and always should be your focus. Again, as has been stated above, there is more than one reason Hughes wrote the poems, and if you were to - purely for the exercise of it - block out the Sylvia references, there is a heck of stuff about the degradation of memory over time etc etc.
I think it would be very difficult to write a TTT essay without mentioning Sylvia at least once. But if you can look outside the incredibly dense Hughes/Plath saga, you can see a lot more about truth and how it is represented in the text. IF after that, you'd still like to use a Plath text, then fine. But if you can't write a TTT essay without using one of Sylvia's poems, does that suggest that your argument is too centred on their relationship?