The purpose of the shield wall is to provide full cover where none exists. The soldier that casts it turns into that heavy cover, but as a side-effect that soldier receives no bonuses. The benefit of using it is to, say, move up to a soldier that is now being flanked and give him some hard cover. Kinda like in the last game with Alloy SHIVs. You could move one up to a soldier standing, and it would provide them with hard cover.
The uses of the shield wall are still very...limited. I think it can have some good uses if the right combination of buffs are applied by your other squadmates, but that's a lot of spent moves that might work better for a different strategy.
Basically, let's say you deploy it in a no man's land of cover and want to move someone else up, say a Sharpshooter, to get in some sweet pistol kills. But you don't want to leave them flapping in the wind and open to flanks, who wants to do that? So you use the shield wall and move someone up into that no man's land.
However, once you do that, then that soldier is flanked. Even though you can now move someone up behind them and they will have full cover, your shield-wall soldier is considered flanked (they're standing in the open). This renders the full-cover benefit to the soldier behind them worthless, because the aliens will prioritize shooting the now flanked soldier in their sights over everything else.
It still seems to me that it might have some interesting things with it, such as a grenadier with blast padding. That gives you 3 or 4 armor pips I think, which is very tanky. Combine that with a very high dodge PCS, and aid protocol, and you'll have a virtually untouchable wall of hard cover that can move. Still, I'd rather spend moves with my specialists elsewhere, like overwatching with guardian, hacking, or using combat protocol. And use my Grenadier for more useful things like Salvo.