Do not forget that the Bugs are able to attack from underground : Troopers in both Heinlein's and Verhoeven's works have learned the hard way that not only can they dig tunnels, even into hard rock, and do it really
fast, they also know how to use theses abilities in a tactically useful way.
I take for example the Clusterfuck of Whiskey Outpost
(in the 1st film) :
Using bug-wave tactics, they forced the troopers to concentrate their efforts into defending the walls of the compound. Hadn't the Fleet came to rescue them, and even if the bugs didn't have the overwhelming number superiority they enjoyed in the film, the troopers would still have been deeply boned, forced that they would have been to cut their defensive efforts into two, dividing the amount of firepower they could direct to each front/attack vector (defending the walls and
the hole in the middle of the compound where the bugs a flowing from).
So, we can see that it would be very difficult to hold static positions on the ground against the bugs. The corollary is that our hypothetical force of Spartan-II would need to be highly mobile, and to keep changing the positions they are defending, while never using the same defensive site twice (or at least not before having destroyed all the tunnels the bugs would have dug leading to it).
The biggest danger come from the immediate vicinity of the PoA drop-site : unless you have a way of destroying their tunnels, nothing really stand in the way of bugs simply surfacing right under
the PoA while leaving you no surface targets to shoot at.
So, maybe the best use you could do of those nukes would be to detonate them underground the Pillar of Autumn drop site to make sure the galleries are destroyed
. Preferably while the bugs are on the offensive, for maximum enemy casualties.