I'm assuming since you're talking about transfer plates that this will be in a wooden subfloor?
Wood isn't a great conductor. You'll have hotter/cooler strips in the floor if you don't use the heat transfer plates and likely have to run higher temperatures. Cooler supply temps can mean more opportunity for condensing at the boiler which is a good thing, if it is designed for it, which is more efficient. Can you have radiant heat without it? Certainly. I've got mine embedded in the floor with a radiant foil sheet and air gap underneath. I think if I was going to do this again, I'd probably use the heat transfer plates, but mine works.
This is all thinking about a low-mass floor. Now, if you're talking about radiant in a slab, that's a different thing...just the pex burried in the slab with adequate insulation beneath so you aren't trying to heat the earth works great. One room has tile, and I embedded the pex in slc. That bathroom, between two bedrooms, tends to keep both of them warm with no additional heat, and, at least in the winter, stepping out of the shower onto the nice warm floor is very pleasant!