If the spaces are really small, as you say, I'd opt for opening up the space. The additional privacy of a separate toilet is nice, but so is not feeling boxed in, especially since the room is used only occasionally.

If the room is small, then make it into a single room. If you were to ask my husband, he would say have the toilet separate. We are finishing the basement and the bathroom will look smaller because the toilet will be enclosed..I had to give in to that:)

I can understand why someone would like to have a private toilet area; but I think in the case of a guest bathroom that isn't used often, your guests would probably appreciate more space and more wiggle room as opposed to a door to block off the toilet area. I don't know how you feel; but when I have guests stay at my house, I like to make things appear as welcoming as possible in the guest room and that means putting out extra fancy soaps and toiletries, and essentials in a basket in case they forgot anything. I think in your case, the extra space in the shower would be more inviting/welcoming, too.

The door to the toilet doesn't matter. It WAS a trend. Knock down the wall and have a larger feeling bath.