I'm sorry...are you talking about ORDAINED deacons, or the "deacons" which you have identified as chanters and readers? Because i've been to a Coptic church which had "deacons" who were 7 years old, ringing some cymbols and bells. So...
These 7 year olds would be chanters. Playing cymbols and leading hymns are duties of chanters, so I don't see the problem there.
Readers can be asked to give sermons, chanters can't.
Deacons can help distribute the Blood, chanters, readers, and subdeacons never do.
Chanters and Readers commonly take the place of the deacon in the Liturgy when there is no Deacon present. They hold the cross and say the deacon's responses at the altar. But they do not help distribute the Blood.
Because they say the words that have 'deacon' written above it in the Liturgy book, many people ignorantly call them deacons. Many mothers then go and buy their little 'deacons' deacon's stoles instead of readers stoles or no stole for chanters. The priest still knows the 7 year old beside him is not a deacon even if he can't tell him to take the stole off without creating a big scene with the parents since it has become so common, so he won't ask him to help distribute the Blood or give a sermon.
Is there something still unclear hear?
The ancient practice is that the Liturgy cannot be prayed without a priest, a deacon, and a congregation. For whatever reason, no one does this any more. The Eastern Orthodox have gotten around it by letting the priest say the deacon's parts. The Copts have gotten around it by letting lower orders fill in for the Deacon. I don't know why we don't just all have Deacons like we used to, but this is how it is now