Transmission Flush and Power Steering Flush

I have a 1999 Acura TL 3.2 with 45,000 miles.

The dealer says it needs a transmission flush because the fluid is dirty. The cost will be $189. He's also recomending changing the Power Steering fluid and brake fluid.

In May of 2004, I had 30,000 mile service which included a transmission fluid drain and refill. The svc rep tells me this is different than a flush and not as good. The cost was not broken out but had to have been much less than $189 because the whole bill was $311. The maintenance book says to go 90,000 miles (72 mos.) Between transmission drains. It doesn't mention flushes.

I've never heard of flushing before now, though I'm really not knowledgeable about cars. I'm wondering if this is really necessary, or a new marketing program--especially since my Nissan dealer for the first time is recommending a flush. This of the power steering fluid in my 1993 Nissan Maxima.

What do you think of doing these flushes?