For me, I was happy to have the valium. My transfers have always been painful due to scar tissue on my cervix from previous Gynae surgery, so it was always excrutiating for me (hence the pethidine/valium cocktail from Wazza!!!).
I'd also experienced cramping and a bit of spotting afterwards on either my first or second Cycle (my original FS recommended valium as well after my first transfer but I had to provide it!!) and although both are quite common after ET, for myself, I'd preferred the preventative approach.
This is another area where FS's have a difference of opinion, same as the immune issue argument, but for me, I think if it's not going to harm/hurt anything (which it's not and it doesn't), then I'd rather give it a go than not.