I don't think that's what is being dicussed. I think that its just people who genuinly wanted to learn Church Slavonic, which is a liturgical language shared by over 2/3 of the Orthodox Christians in the world. Its like saying NT Greek won't do you any good. Of course you can't speak it, but are you ever going to truly understand the NT without it? Not likely.
As for Serbian and Church slavonic, its not really the best way to go, to be honest with you. If you learn Serbian its going to be harder to learn Church Slavonic, just because of pronunciation. Serbian is a completely, and I mean COMPLETELY phonetic language. No hidden gimmics, what you see is what you get. Church Slavonic has abbreviations out the wazoo. It also has combination letters, accentuation, and inflections. Don't even get me started on the grammar. Russian is going to give you a better idea of HOW to pronounce the words you're going to see in Church Slavonic.
On a more personal note, I prefer the Serbian pronunciation of Church Slavonic, it sounds a lot cleaner. However, that doesn't necessarily mean that we're pronouncing it CORRECTLY, according to the accentuations and grammar, etc. So, all in all Russian is a better stepping stone for the Church Slavonic. If Russian ends up being too hard to learn, its very easy to switch to Serbian, which might end up being easier for you.