What runes to expect? Low ones. In terms of the highest rune that chest can drop, you're looking at Vex. In terms of efficiency neither the Pit nor AT sparkly chest are worth opening since both their TC cap (TC66 and TC69, respectively) and rune drop rate/cap (Vex/Lo) are too low.
Not everything good is from high armor and weapon TCs. The chest can probably drop nice jewelry. I wouldn't call the sparkly chext in the Pits it a really shiny highlight, but I would not leave it unopened. I would even take risks to achieve it. Where I am, all loot is MINE! It's a matter of principle or even of spite, not one of efficiency or reason.