As for those converting from Christianity to Islam, that only happens with people who were very nominal Christians - those who were not committed to Christianity at all in the first place.
Funnily enough, that sounds almost exactly like what is written at your link about Russian Muslims converting to Christianity:
According Silantyev, the converts are predominantly Muslims by birth, while ‘those who really confess Islamic values and attend mosque on a regular basis rarely change their faith’.
So, if you and Mr. Silyantev are correct, the same thing applies both ways - the ultra-commited in any faith tend to stay there, while the rest can flop around willy-nilly between religions depending on social pressurs or whim. I don't know if this is true, from personal experience it seems to me that some people have a tendency towards fanatacism, and have no problem swinging from one fanatacism to another, but if it is true it's probably true of all faiths, rather than just Christianity.
You also have to realise here that your own faith may make it hard for you ro interpret the evidence you see. Because you're firmly committed to the truth of Christianity, and you know in your heart of hearts that it's right, it seems intuitively wrong to you that someone can genuinely believe the tenets of Christianity and then switch to another faith. You're already primed to see converts away from Christianity as not being real, genuinely believing Christians.
Well, the funny thing is that those of us who believe what the Bible says understandably care if people are going to Heaven or Hell. It matters to us and encourages us to see people freed from the demonic chains of enslavement to a Satanically-inspired violent belief system (inside the Kingdom of Darkness) and instead delivered into the Kingdom of Light - loved, forgiven children of their Creator God.
It's exciting to see what God is doing. In a lot of the cases, from what I have heard from missions sources, God is appearing to a lot of these people in their dreams at night, telling them over and over in their sleep, "Jesus is the Son of God."
Last night I dreamt that my girlfriend had dyed her hair really light blonde, and, depsite thinking it looked stupid on her, I had to pretend that I liked it to avoid upsetting her. Why's god forsaken me?
On a more relevant note, I think you misunderstand what Straggler meant when he asks why it matters. Of course, if your interpretation of Christianity is correct, then it matters whether people accept Jesus into their lives, since this determines theit eternal fate. What he was asking, however, is whether people converting from faith A to faith B is evidence for the truth of faith B. It could just mean that faith B is more attractive, or has better missionaries, or it could just be a reflection of some particular social pressure, or just arbitrary drift.
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I don't think the data actually exists which would enable us to judge which faith is converting more of the other. It's not really collected systematically, and census data on religion is open to all sorts of problems. As you mentioned, conversions might well be hidden in a strongly Muslim country to avoid persecution, people may hide their beliefs from family and friends to avoid ostracism, and people mean different things when they answer 'what religion are you'?
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