Like others, it does seem strange for a therapist to suggest this. Is there any reason why she might have thought it would be best for you to have therapeutic support during birth? Perhaps to it more bluntly, do you have a history of becoming violent towards yourself or others when in a dissociated state? that's the only sensible reason I could come up with.
The other question I have, is whether your husband is supportive, and do you feel he is knowledgeable enough about PTSD to know how to support you by informing people that you may be dissociated and less responsive to them?
A mildly dissociated state is not a bad place to be during birth, it is perhaps normal. Just my own experience - I remained dissociated after the birth and I experienced fainting attacks that were said to be psychological shock. After my second child, I fainted and collapsed, and my midwife was irritated (sympathetically) that I hadn't told her i was feeling faint. the reason I hadn't told her is because I was very dissociated and in a different world. I think this is the time when it would have helped to have had someone there who noticed dissociation, and could reach in and call me back, and get me talking outwardly, rather than not realising I was shut off.
If you have a birth plan, it is perhaps worth noting the basics in there - that you experience anxiety, dissociation etc.