Well, both in the control room and on the twin bridges where it's used at the ends, the material can provide cover from hostiles, which could be why Bungie wanted it indestructible. Maybe the shattering effect was there for a bit of engaging reactivity, and they just didn't worry about how anomalous it is.

In the control room the material also serves as safety barriers, so again Bungie might not want it to vanish (the bridges are a bit different as there's low walling below the panels). In the one place where there's no barrier, namely at the control panel, I remember going off the edge loads of times when attacked by the Sentinels after the cutscene. That used to annoy me a lot. Falling to my death within two seconds, great!