Specificity, or my impression of it, really is marginal gains. So if you’re at the sharp end of a race, you need an edge and the bike specificity really is your limiter then sure, I get it. Otherwise I just don’t see the point.
If you’re going to fully lock it out then you may as well be on a rigid anyway. Gains on the trainer, on a rigid will be more than transferable, unless, as I say above, you’re really looking for those marginal gains.
Just my 2 pence 