You can get more of them in a tube. I shot a round of trap with the birdshot loads the other day. I normally average in the 20s, shot an 18 with them. I shot another 5 rounds letting the targets get out to about 40 yards. I moved targets twice, but broke only 1 of 5.
The slugs/buck might have some purpose if you want to stuff 8 in a tube as opposed to 4 regular slugs/buck. I did shoot some mini-slugs at Hogs about 7 or 8 years ago...they worked just as well as the 2 3/4" slugs. SD shootings with shotguns are not that common and you have to look pretty hard to find any SG uses in legal defense of life with more than 3 rounds.
frankly, if you need more than 6 rounds of 00 Buck.....you are in some serious shit.....
and honestly, i would rather take the time to reload my shotgun.....even just keeping it topped off as i shoot, which is really easy to do with a tube fed gun over relying on the mini-shells feeding properly.....
now if they designed a shotgun to reliably feed mini-shells...especially a semi-auti...then that would make them much more viable.....and i would happily trade reduced power over greater capacity.