Your criticism is fair, however, not everyone keeps their Android phones configured thus. A growing minority of users--whom are the target audience of software like this, I suspect--maintain sole root access over their phone, or otherwise restrict privileged access from other software on the device. There do exist Android devices where no-one but the user have control, even without custom roms or 3rd parties at play. (Firmware is a separate issue, but that goes well beyond smartphones, and even then some phones perform hardware isolation by way of iommu.)
But aside from this, hidden services aren't exclusively used for the clandestine nor the malicious. It's a way of port forwarding on any network, even through a NAT, while also anonymizing traffic on both ends, for free. That functionality alone goes well beyond the purpose of simply hiding data. See Briar[0] for a particularly well-executed example of this.
[0]: https://briarproject.org/