Slabbing does not offer "perfect" protection for coins. The grading services have had to buy back coins in the graded condition after they deteriorated to a lower condition while in the slab.

The main reason for slabbing coins is to have an independent 3rd party assign a grade to a coin. This is important in the coin hobby because you have to be an expert to accurately grade a coin as well as to detect alterations, counterfeits and otherwise fraudulent coins. This provides some protection to casual and even serious collectors, but also to investors. It sounds like a good system on paper, but in reality it is a nightmare. Grading standards change over time and are not consistant from one type of coin to another, or even with in the same series of a coin from year to year. For example, one Morgan silver dollar graded MS-65 may look entirely different from another one in the same grade. So even with slabbing, you still need to be an expert if you don't want to be taken advantage of by a less than honest coin dealer.