In the arid West, with its hard soils, it's a lot of work to dig an adequate cat hole using a branch or hiking pole. Even in country with softer soils, not carrying a trowel encourages people to quit digging before their cat hole is deep enough. The big excuse for not carrying a trowel is to save weight, but that excuse no longer applies.
For years I've carried a sturdy plastic trowel on all my hikes, as part of my commitment to ethical hiking. For something I used so rarely it was on the heavy side, 2.95 ounces (83 grams.). Now that trowel is in a bin of garden tools, replaced by an anodized aluminum version called the Deuce of Spades or just the Deuce. The trowel weighs just 0.6 ounce (17 grams), comes in multiple colors, and costs $19.95 at REI. If you're still hiking with a plastic trowel, the time has come for an upgrade. If you don't hike with a trowel, it's time to start.