Before my time, me old fruit.
Look it up and you'll see what I'm on about. Mini army vs. full army with hilarious mission twists for a bit of game balance.
So why not use the formation?
That means they work as a unit which is lame and completely defeats the purpose I would have in mind for the models should I come across someone looking to use them in games.
...like hell I'm buying something that has an Imperial fucking Fist with a frag cannon.
It's not about having a full sized army, it's more about themed game play among those nostalgic enough to remember the WD rules/missions for the Last Chancers.
2+ squads of EITHER Sternguard or Vanguard, in any combination.
And the anti-phasic bolts only affect bolters/combis, so no help to pistols in the hands of the Vanguards.
There's also bonus VP if these guys slay a Xenos warlord.
Meh, that's not too appealing to me. If I was gunna do a Formation of Sternguard/Vanguard I'd bust out the modeling skills and concoct that BA Formation where everyone gets free power (VG) or combi-weapons (SG).