The simple answer to that is NO. Sometimes contaminants could have bleached the dye from the fibers of the carpet, replaced the dye in the fibers, or just stained the carpet fiber. Just like a stain that you may get on a shirt or pants. Most carpet cleaning companies out there do a good job in the cleaning process and there is not a better technique or process than another. They all have their place in carpet care.
It is only necessary if you plan on keeping your carpet cleaner longer. It only last a short period of time and will have to be re-applied every time you have the carpet cleaned. Wear on the carpet, spotting agents and other cleaners, soften the bond of the protector from the fiber over time. Therefore it looses its protection capability. Professional carpet cleaning companies applied the same stain guard protectors that are applied at the factory and many manufacturers require that it be cleaned and re-applied every so often to maintain your warranty so make sure that you keep your receipts.
Yes, there is a possibility. This is called wicking. Just like a candle wick re-emulsified liquids beneath the backing of the carpet can become wet again from the cleaning process and wick back into the fibers as the carpet dries. There is no way of preventing this in most residential settings but commercially this can be avoided frequently by using the encapsulation method.
This all depends on the amount of traffic and the amount of soil brought into the home. Under normal circumstances (3-4 person family), we recommend every other day - once per week depending on whether or not shoes are worn inside or not. If there are pets and more people in the home a daily vacuum may be required to reduce the amount of dry soil filtering and other contaminants sticking to the fibers.