Meh, unless you´re wildcatting to something like 22-243 it has no sense or value whatsoever.
Keep in mind there is a limit to brass springback, which in turn means a limit to neck tension as we use to refer to it. This is roughly up to 3 thou on std calibers, maybe 4-5 thou on 338 and bigger. Anything more you´re just uselessly moving brass to another diameter. Conversely to that, if the brass were able to springback more, it would not be possible for reloaders to neck-up or down in general as the brass would always just spring back to what you started with. But that doesn´t happen.
All that happens when you size the neck down any further is that the bullet has to act as an expander. Depending how far under diameter you go, it may not seat at all and crush the shoulder, seat crooked etc. However, if you do manage to seat it, it will not be held any firmly than with regularly sized brass. If you really need to increase the force holding the bullet in place, crimp it.