Taking a laptop through airport security as a carry-on doesn't involve letting it out of your hands. (I assume they do not hire hobbits to hide inside the X-ray machine and implant hardware on your motherboard.)
In any case, just because you usually get your laptop handed right back, it doesn't follow that you always will. If it matters to you, buy that burner.
1. I can expect that there's a chance that my laptop will be taken from me (and perhaps returned modified or perhaps taken permanently) while still not being okay with that. It's one thing to expect a right to be violated; it's another thing to give up.
2. It's not unreasonable to travel with a laptop while expecting a small chance that it's taken, in which case you'll buy a new one and restore from backup, instead of buying a burner every time.
2. If you truly believe your laptop is clean and encryption sufficiently uncrackable, go ahead. I don't think I'm that smart, myself.