I'm going to apply hindsight in answering your question Jessica. But first I want to say I once asked the same question, as going full time, without having been on HRT for a while, made no sense to me.
Now I know HRT can affect your thinking. And it can take whatever inclinations one has about living life in the opposite gender and make them stronger. In other words, if you're on the fence, HRT can push you over it. So HRT can create a situation that's not truly congruous with one's personality. And that can result in a false positive.
Sure, if you transition you will most likely remain on HRT for the rest of your life. But that logic is like saying if you place yourself in a depressing situation you'll be okay, as long as you take anti-depressants the rest of your life.
My hindsight tells me you must first KNOW, not guess, not suspect, but actually know you need to transition. But if you don't know for sure and want to know, why not do the RLE to find out? No doubt, passing will be tougher. But if you still want to transition after that, you'll really know for sure this is right for you.
I realize going on HRT is a great way to experience a little of what it might be like to transition. And most can do it for a while without anyone knowing. Kind of like our little secret, and behind closed doors or out on late evening jaunts, it's great. But it does little in preparing you for the realities of transitioning and usually increases your belief transitioning is right for you. There's an element of illusion living this experience.
I look at my partner Julie. Hands down, without a doubt, I knew transition was right for her. And today she will tell you it was. I can't say the same for me. As time goes on, I am finding the pendulum swinging back to male more and more. I'm also getting tired of all the work I have to do to pass. And I think I've ruined my voice. I'd categorize myself as one of the fence sitters that HRT pushed over.
Now I can see the logic in doing the RLE before HRT.