Myself, i don't believe there is much of an advantage to these conversion, if one is only going to use BH. Like you wrote, there could actually be a disadvantage.

Some have written that the small rifle magnum primer will be reliable igniting BH. I do know the standard small pistol primer will not ignite BH. I have never tried a magnum primer, so i have really no idea if they would work. By using these conversion one could make a plug with a huge flame channel, if one so desires. Perhaps this would be an advantage. Another possible advantage, may be that it would be easier to 'handle' the brass in the field, than a 209 primer. I have dropped 209 primer into the snow, a time or two.

If i run out of chores to do this winter, i probably will make a breech plug to accept 25acp brass, and test it with BH, using small rifle magnum primer. However, i haven't ever run out of chores very many times in the past.