Shelley I have to disagree with you on this one - there is such a thing as transient microbes. Specifically any flora which doesn't take up permanent residence but is outcompeted by other flora. An example would be S. Boulardii - the tropical yeast (an ingredient of Primal defense). This establishes and outcompetes candida initially, but is itself outcompeted by the body's natural flora. The only way S. Boulardii can survive in a gut long term is if there is a complete absence of normal flora (which only ever happens in lab test animals which have their flora deliberately wiped out for test purposes).
Otherwise the solution to candida would be easy - just take S. Boulardii, which performs the same function as candida (in terms of breaking down rotting food), but instead of producing toxins produces valuable B vitamins. Unforutnately it only works whilst you are taking S. Boulardii - it being a transient strain.
People stop feeling the effects of PD because it consists largely of transient flora - S. Boulardii, Soil based organisms, and according to Duncan Crow (a holistic healer who posts on the yahoo candida board) it even contains an animal based strain. Certainly you won't find these in other probiotics. And you wont find them as natural inhabitants of the gut.
Bottom line is that PD consists mostly of HSO's - homeostatic soil organisms. Organisms which live naturally in the soil, not the human body. Whilst they can do much good in the human body, ulitimately HSO's will be outcompeted by other flora.
The final proof is in the pudding. Take primal defense and you will notice big changes. Stop taking it and your body will quickly revert to how it was.