Car Experts: To flush or not to flush (the transmission)?

I have been looking up transmission flushing on the internet and some sites advise against it, citing potential damage to the system (transmission) as well as engine. They do suggest a full transmission service (drain the fluid, check the pan, refill, etc) but not a flush. However, two mechanics I spoke with, including a dealership are “pro-flush”, one even boasting that his shop bought a $5000 system. This kind of goes along with the warnings from the web, which say that shops buy the expensive equipment and try to convince customers they should get a flush.

So, are flushes necessary, or a waste or time. Or worse: potentially damaging to the engine/transmission? What say you?