The newer Empire Detachment box (no longer in production), with the melee and ranged infantry, was a very tempting buy. Alas and IIRC, already back in 2009 it was out of production, and I couldn't find it locally. Didn't it also contain a Helblaster?
About halberdiers and shields, I have two, no three approaches. Two of them historical:
Historically, late medieval/renaissance infantry with halberds and pikes used the shields while advancing, and then either threw them away, or hanged them on their backs when engaging the enemy. So the shields were there to protect against arrows. Some units/armies had separate shieldbearers for the task. These could be the same guys who also fired the crossbows - and indeed, skirmishing crossbowmen with shields were the norm around the 14th and 15th century (very roughly). The shields used for this purpose, in this kind of formations, tended to be very big and rather rectangular, real pavises, sometimes with a pole for support and a pike at the bottom to lock them on the ground. No GW shields produced for the Empire ever had the proper size & looks for these kind of shields. 6th/7th ed Bretonnian infantry had such pavises, and the largest knight shields (also sold with some Bret infantry boxes) up to the 5th ed depict the earlier, more heather-like large shield.
The shields that come in the State Troop box are target shields (Tartsche in German). These seem to have had an Eastern origin, and were mostly used by cavalry. They started to appear in the 14th century (or even earlier in Eastern Europe). In the 15th century, the last century knights really used shields, target shields had become the norm. They were of many forms, but were mostly rather rectangular, with holes or other forms to support the lance or allow some tricks against the enemy weapon. This led to new forms of the heraldic shield too. Many GW fantasy shields echo these late heraldic shield forms, which were really never used in battles. So the State Troop shields are not very well suited for infantry fighting in formation. Some swordsmen detachments did carry such smaller shields, but they often chose round or roundish shields (the Spanish swordsmen for example). The State Troop shields might be better suited for cavalry than infantry, and you might want to look for other shields by other manufacturers for your infantry.
But who cares of history? It's a fantasy game.
In game, it's more about the look and the rules, and if you want to say your infantry has shields, better show it too.
All the State Troop bodies with breastplates do have some sort of strappings on their backs, although some of them are really thin, just lines on the garment. IIRC a couple have real straps. I don't think it too far fetched to say that these straps can be used to hang the shield. Carrying a small to medium shield on the back while using a two-handed weapon was practised. It will look good enough.
Some interpret the small target shields as buckler-like, and actually glue them on the forearm of the halberdiers (or the spearmen). Personally, I think it looks awkward. The shield doesn't protect much more than the forearm, it looks too heavy to use with the polearm, and it obstructs sight. Real bucklers were smaller and usually round. I don't recommend this option. But who am I to say what people do with their fantasy minis?
-Z