Oliver Cowdery, Martin Harris, and David Whitmer are known in LDS history as the Three Witnesses. These three were the three major witnesses of the gold plates and the angel, and at times assisted Joseph Smith. From what I have experienced thus far in defending the testimony of these three are attacks primarily on their motives. The other eight witnesses don't really come into the picture, as the Three Witnesses are the bigger target.
These three never denied their testimonies throughout their entire lives. Even David Whitmer, who never rejoined the Church after his excommunication, testified of what he saw at his deathbed to his family. While being honest about what they saw doesn't really sit comfortably with anti-Mormons, the idea that they had ulterior motives does. Let's take a look at a few of these motives and see if they are valid or not.
The first one is, of course, wealth. Some suggest that there was a lucrative quality to their testimony. However, if they were any good at being con artists, they would have instantly realized that there were ten thousand easier ways to dupe the population than professing to have seen an angel and gold plates. Joseph Smith's claims of a "golden bible" were ridiculed by his townsfolk even before he received the plates. It wasn't exactly appealing as an investment.
The Book of Mormon never sold well, and the majority of the population thought it was a "gross imposition, and a grosser superstition." All signs pointed away from the possibility that they would get rich from it. And after the Three Witnesses left the Church after their own manner, wouldn't they have ratted out Joseph Smith if they were looking to get money out of it?
Instead, what they got was scorn and ridicule. David Whitmer was once threatened with this life concerning the validity of the Book of Mormon. Perhaps some people would like to think so, but I do not think that David Whitmer would have stuck to his testimony for the possibility of a check at the end, as opposed to the possibility that he would be killed.
Another possibility would be fame, but what fame would they have gathered other than notoriety? They were threatened with their lives.
And then there's power. When the Church was founded, the Three Witnesses were given hardly any power, and it was taken away after they left the Church. The "power", or authority, that they had prior to excommunication was not given back to them after two of them returned. Yet they still returned.
A quote from Michael R. Ash from his FAIR Brochure concerning the Book of Mormon witnesses and their motives:
"David Whitmer- like the other witnesses- had been charged with being deluded into thinking he had seen an angel and the plates. One observer remembers when David was such accused, and said:
"How well and distinctly I remember the manner in which Elder Whitmer arose and drew himself up to his full height- a little over six feet- and said, in solemn and impressive tones: 'No sir! I was not under any hallucination, nor was I deceived! I saw with these eyes, and I heard with these ears! I know whereof I speak!'
"Could the Three Witnesses have been so caught up in the excitement that they imagined they saw an angel, or lied about seeing an angel to heighten the stimulation? An affirmative answer may suffice for testifying 'in the heat of the moment,' but this explanation is not satisfactory when we look at the testimony that they continued to proclaim throughout their lives- through prosecution, financial ruin, excommunication, embarrassment, and bitter feelings. A testimony born in the excitement of the moment would die quickly under such adverse conditions."
That sums up nicely what I've been saying.