The steam turbine (invented by Hero of Alexandria in the 1st century BC, and improved dramatically since then by Parsons, among others) is a kind of steam engine. With regard to reciprocating steam engines, I don't know of any in current use either, but I would be surprised if there weren't at least a few niche uses.
You are certainly correct about high-pressure steam being essential. Did Watt's patent make it illegal to develop high-pressure steam engines, or just uneconomic?
The (practical) steam turbine is one of the fundamental, yet unacknowledged, bases of our material civilization. Parsons based his intricate design on the equations of thermodynamics; he succeeded in a very complicated task where many others had tried, and failed. His work is about as many orders of magnitude more complex and demanding than Hero's aoelipile as a modern race car is compared to a spring-powered toy cart.
That said, I agree that the terminology is flexible and the steam turbine can be considered a kind of steam engine; it is, however, very different in its principle of operation from the Watt-type steam engine (which it displaced in the XXth century). Regarding high-pressure steam, as far as I know Watt's patent precluded anyone from using a condenser in their steam engine, which is an essential component. Thus only engineers affiliated with Watt himself were able to produce commercial steam engines during the 25 years he held the patent, and he refused to let them work with high-pressure steam (despite a good deal of interest).
For a more thorough, and much better documented, discussion of these issues, I recommend chapter 3 (Internal Combustion Engines) of Vaclav Smil's book "Creating the Twentieth Century".
It certainly is a very important invention, but I'm not sure what you mean by "complex and demanding"; you can build a perfectly good steam turbine with one moving part plus a high-temperature thrust bearing.
A condenser is not an essential component for a high-pressure steam engine. It can be useful (modern power-plant steam turbines use condensers because they're quite sensitive to lime deposits, so the water has to be expensively purified before use) but e.g. steam locomotives did not normally use them.
It is not the case that there were no non-Watt commercial steam engines built in England during Watt's patent; a good number of Newcomen-type engines continued to be built. But thank you for explaining that Watt refused to license his patent.
Thank you very much for the book recommendation! It sounds like a book I've been wanting to write. Maybe Smil has saved me the trouble.
It's interesting that reciprocating engines are quite common in the modern economy outside of the steam department. There are even commercial electric power plants driven by reciprocating engines. But they're internal-combustion.
The concept is relatively simple (once someone thought of it...), it's the execution that's difficult. (For a simpler example, let's look at breech-loading firearms: the idea appeared almost as soon as gunpowder, but it took until the 2nd half of the 19th century to achieve cheap, practical designs.) In the Parsons turbine, the shape of the components is much more complex than the piston plus cylinder of the Watt engine (see http://www.leander-project.homecall.co.uk/turbines.html for a diagram). All the little blades and such also need to hold up to high pressures and temperatures.
I'm not entirely sure about the condenser bit, but I do remember Smil addressing the issue with references and all. His books are quite amazing, it's quite a pity they're not better known.
You are of course correct that the Parsons turbine is quite complex and demanding; I didn't realize how fully he had developed the concept. (I still take issue with "as demanding as a [modern] race car", though.)
However, simple rotary pneumatic motors are used quite effectively in many hand tools, and they're essentially low-speed turbines. They don't have the immense efficiency of the Parsons turbine, but they are quite simple and can be quite reliable. (The high-speed ones used in dentists' drills had to wait for the development of the air bearing, though.)
You are certainly correct about high-pressure steam being essential. Did Watt's patent make it illegal to develop high-pressure steam engines, or just uneconomic?