Well, I suppose that I am not going to win any argument with you on the finer points of what is, and is not donatism. All I know about it is that it was a breakaway movement from the RC Church (I think in Africa somewhere), and that it was characterized by the requirement of special moral purity among church leaders. I also have the impression that it is looked down upon by most people today for one reason or another.
So, what is important to distinguish is whether Baptists and donatists are really the same thing, or whether they simply share one, or a very few similarities. (Catholics and Protestants believe Christ is the Son of God. By no means are they the same faith.)
First of all, we have no "bearing fruit" test for baptism. I've never even heard of it among other Baptists.
If requiring that a formal member of a Baptist church actually be a believer in Christ somehow provides a fair comparison between Baptists and donatists then so be it. I don't know, but I do know that it is ridiculous to criticize a faith for requiring that members actually believe. How does this work in the Catholic Church? Could I just waltz in and be welcomed as a member, even if I held to my SB beliefs? My instinct tells me 'NO WAY'.
In fact, I'll bet that an outside candidate for membership in a Catholic church has to jump through a thousand more hoops to join than a similar candidate at my church. Which is the church of elitism?
At base your observation about the Donatists is a good one. Fundamentally, why would Baptists care if they are Donatists? Whether Baptists are viewed as a new form of heretic or a reinvigorated old form by The Church doubtless makes no difference to them...which is why they shouldn't pretend they are The Church, rather accept that they are something else entirely which is good enough for them (ecclesial assembly?).
It is OK to ask for communion in faith before allowing communion in the sacraments, I agree. Specifically, a Catholic must be baptized and confirmed in faith before receiving the Eucharist, and whenever he is burdened by mortal sin, he is to confess that sin before receiving. (Or intend to confess at first opportunity when confession is not available prior to Communion). In addition, a Catholic recites the Creed prior to Communion, although not necessarily at every Mass. However, there is no test of purity, and in fact the Church understands herself as a ministry to the sinners. When a criminal, an addict or an adulterer comes to Church, she rejoices all the more because her obligation is to serve that man and assist him in reaching holiness.
Donatism is the notion that baptism is invalid unless the minister of the baptism is morally upright -- has fruits of the faith, as I put it. Since baptism is acceptance into the church, it means that the church membership is restricted by circumstances other than the will of the baptizee or his sponsors. My understanding of the Baptist theology is that the baptizee must profess faith whereby he is saved. That means that a sinner who does not consider himself a regenerated, sainted person cannot enter the communion of baptists. I admit that the considerable difference is that the baptists do not have priests, so while donatists place the restriction on the person of the priest, the baptists place the restriction on the baptizee, who is sort of his own priest at baptism. But the outcome is the same: the church membership is barred to those who desire regeneration but have not achieved it.