On the LSV, I believe there is actually a lock-ring that prevents the stem from being taken off the heating element between hits. This means that you are going to be vaporizing THC into the atmosphere between hits. Probably not much, but some.
You believe wrong.
The only thing you could keep attached is the WPA, and you'd have to be using it against the manufacturers instructions.
It has a little ring that you can unscrew when using the WPA, but you are supposed to leave that off when using it, you could screw it back on, but it'd be wasted effort because the bowl would need a stir after a hit or 2 anyways.
Also, even if you managed to leave the WPA or mouthpiece connected, there is extremely little heat transferred without you drawing.
I can put my finger inside the spherical joint of the LSV when it's been on for hours, and it's basically just warm...
To be a bit more on topic, Logs are generally considered efficient (in this context meaning reduced overall herb consumption) because they can deal with very small loads.
Most other vapes can handle as small a load as most logs, but the hits are usually pretty wispy.
If you had a butt-tonne of self control, there's no reason you couldn't be as efficient with just about any vape, but a log forces your behavior to go down a path that will lead to less consumption.
My average load in my LSV is between 0.1 - 0.2g.
That could be as many as 8 PD stems.
I might've been happy after 4 stems, but since I vaped that + some, my consumption is higher.
You should also note that logs aren't going to magically force you to use less.
My consumption didn't really change much when using logs, I just had more stems