Personally, I use PIAT teams the same as I use Bazooka teams: Primarily as Recee or Scouts. The absence of dedicated or adhoc recon teams in the order of battle for almost all of the battle groups is a significant departure from the CC5 order of battle. The designers have said that airbourne elements did not have scout sections, but that ignores the adhoc reorgizations done on Day 1. Eh. Book learning versus pracatical application under fire. That does not explain the lack of Recee or Scout teams in Infantry divisional units, however.
Under the current ORBAT, I use PIAT and Bazooka teams as Recee / Scouts. I place them if needed to counter tanks, but primarily use them as fast moving throw-away units to gain ground, clear unexplored terrain to deny that ground for enemy placement on any further battle and to press my territory claims into the enemy's flanks, middle and rear. It is a shame to have to expend AT units as scouts, but there is no other choice in the TLD ORBAT.
If you are up againt tanks in an urban map, PIAT's might be of some small use. A MMG team or any team with a Gammon bomb will probably serve you better.
In bocage or forested terrain, PIATs and Bazooka teams are best used as scouts or replaced with heavier fireprower units.
C'est magnifique, mais ce n'est pas la guerre.