PV is generally superior to solar thermal because it produces a higher-quality energy: electricity rather than low-grade heat: 1kWh of electricity has more exergy than 1kWh of low-grade heat. As you say, PV's efficiency tends to be significantly lower than solar thermal, in straight watt-for-watt terms, which means less power per unit roof area.
And you're right that just using the electricity generated to create low-grade heat could be wasteful.
But exergy and economics are different: PV is becoming cheaper and cheaper. So maybe there are circumstances where PV + resistance heating is cheaper than solar thermal. Misaligned incentives between PV and solar-thermal could make that happen, as could extremely cheap PV systems.
Deemed exports within Feed-in Tariffs
And there are cases, for example in the UK, where the payment to the PV system owner is based on the amount of electricity generated, and where there is a payment for electricity exported to the grid, but this is not measured - it's just deemed to be 50% of the total generated. Yes, that is a silly way to do things, given how cheap and easy it is to meter the exports, but that's the system some utilities are operating.
Under such a regime, the PV is then built with a resistance heater in a hot-water storage-tank, and all the electricity gets dumped as low-grade heat into that tank. Dumping it as heat doesn't change the revenue generated for by the PV system, and does reduce the heating costs of the homeowner, as well as given them very-low-carbon hot water.
Hybrid photovoltaics + solar thermal
Finally, as Highly Irregular notes in the comments, there are various PV-thermal hybrid systems, which provide both electricity and low-grade heat: in those cases, some of the PV electricity is used to pump water, but the water is heated by the sun directly, rather than by resistance heater - and this cools the PV panel, raising its electricity output:
There are people trying to combine a PV-thermal hybrid with a heat pump, but the economics really don't favour it at the moment, and current heat pumps can be pretty noisy, particularly in their defrost cycle. A ground-source heat pump + PV-thermal hybrid might be superior to just a heat pump on its own.