There are plenty of conflicting ideas about a transmission flush being necessary. Most mechanics will suggest that if you have regular engine servicing, oil changes and fluid checks, a flush is not needed at all. Even if your transmission is not gearing correctly, it is not necessarily a call for a flush.
Regular engine and transmission maintenance will mean that your vehicle stays in good shape. With newer vehicles having parts which last far longer than their old counterparts, there is little reason for flushing a system these days. Owing to the longevity of these parts, it makes it more difficult for mechanics to earn a living and therefore some less scrupulous ones will suggest a flush just to get the money when they know it could cause damage.
Flushing the transmission fluid from your vehicle can actually harm it rather than help it. There are some schools of thought which claim that it is similar to changing oil. When you put fresh oil in your vehicle it is clean, but over time, it will break up and become far less effective in your engine. Transmission fluid will eventually have less effectiveness, but the dangers are that flushing causes too much pressure to run through the system, causing irreparable damage to the fine and minute course it takes through your engine.
In reality, transmission flushing is not necessary and not recommended. In fact, many manufacturers put a disclaimer in the auto manual and suggest you do not flush the system.