Reading Horton and other Reformed systematic theologies this year, I've noticed something that I think one could refer to as "Pneumatological Arianism." The Reformed paranoia is that if the Son were really present in Word and sacrament, then God's power and transcendence would be lost. God would be under our power, he would be objectified. This would prevent us from looking upon God's bigness, power, and glory with proper respect.
The problem is then: How does God's grace come to humans? Answer, although Jesus is gone, the Holy Spirit serves as his mediator. The mediator then needs a mediator and we are stuck with an pneumatological Arianism. In other words, for the Reformed, the Holy Spirit can come to us without somehow abrogating the transcendence and power of God. But why can't the Son simply come to us? Setting aside the question of real presence of the humanity of Christ, Horton seems to deny that the second person of the Trinity can have any direct union with us. Nevertheless, both the Son and the Spirit are equal in divinity, so what gives? Unless they weren't! Though in theory the Reformed hold to the Nicene creed like the rest of us, in practice they treat the Spirit as a sort of lesser go-between with the rest of the Trinity and us. Again, Horton inadvertently seems to say something like this when he says "The Son is not in direct union with us, but rather dwells in us by the Holy Spirit."
In effect, both position spring from the same soil. Arius was also afraid that God's would lose his otherness if the Son were really God and he really became incarnate. The Reformed simply move things down the line: the humanity of Jesus and the Holy Spirit are the new go-betweens.