I have a 2002 Subaru. I took it in for its 30,000 mile check up today, and was told that I should have the coolant system, brake system and transmission flushed. Is this really necessary, I thought that I had heard, somewhere, that these flushes, especially the transmission and brakes, do nothing more than waste fluids?
We had several posts on this in he last few months. At that mileage, I would DRAIN the transmission and change the filter (not flush),flush the cooling system, check themostat & water pump. The brake system is normally drained when work is done on the brakes, around 50,000 miles or so, unless your owners manual says otherwise.
In all cases, READ YOUR OWNERS MANUAL; it overrides service tech recommndations.
On a Subaru, maintaining proper engine temperature is critical since this engine is famous for head gasket failures, usually a result of overheating. This will cost you megabucks if it happens.
Your owner’s manual might recommend cooling system service every 50,000 miles or every three years. Same for the brakes. The dealer will recommend such services to everyone who walks in the door.
Transmission service is important, however. Specify the drain & filter service rather than the flush. And, as Docnick emphasized, read your owner’s manual. That book trumps the dealership.
You should do whatever the owner’s manual says to do. The dealer or mechanic has a vested interest in selling you more. Fluids do need to be changed regularly and that includes the brake fluid and transmission.
Note: Changed does not equal Flush. (sort of for the radiator however) Flush is a new term for flushing your wallet. 
It’s good to replace the brake fluid in most cars periodically to get rid of the by-products created by the brake fluid’s tendancy to pull in water, but if the O/M doesn’t reccomend it you probably can do without it. They use the word flush because that’s about the only way you can replace the brake fluid. Question for anyone that knows: Why the heck can’t they make a silicone based fluid that meets all or most automaker’s specifications? Why does it always have to be polyglycol? The stuff pulls in water and lowers the boiling point.
Don’t flush the tranny. A simple pan drop and filter replacement is better. Transman and others have said about a million times on this forum that it gives the mechanic a chance to replace the filter and clean out normally occuring metal shavings from pan and also to maybe see a problem that could be nipped in the bud.
If you’d bothered to read the owner’s manual you’d know which of these “services” were necessary and which of them were profit generators for the dealer.
Take the manual out of the glove box and READ it. You spent a lot of money to buy this car. Read the manual and learn how to take care of it. It will save you some money.
Just do a drain and refill of the coolant (5 years old, it needs it) and the transmission fluid (automatic, not if manual). Do not flush either of these.
Old brake fluid absorbs water and can make the brakes less effective. You could drain and refill the brake system, but that would introduce air. A brake flush is where new fluid is added to the reservoir as the old fluid drains out keeping air out of the system. So do have the brake system flushed.
The fluid in automatic transmissions runs hot, and heat brakes down the oil (transmission fluid). A flush is supposed to work like a brake flush, but in reality it doesn’t. It won’t get out 100% of the old fluid because old and new are constantly mixing in the sump. A drain and refill will not get out all the old fluid, only 40-60%, but if done every 30k miles, that enough. It will keep the oil fresh enough for the transmission to last the life of the vehicle, or a reasonably long time anyway. It might give out if you keep the vehicle for 250k miles or more.
Flushing of the cooling system and automatic transmissions is rarely needed, adds considerable cost because of the equipment involved and IMO leads to damage to these systems. It does more harm than good.