Not because I support it, but because in the circumstances in question the implication that they should have been treated as "full people" (in the only place 3/5 applied) is the pro slavery position.
Of course the slaves should have had freedom. Of course they should have had the vote. But they had zero of either of those. Increasing the power of the society that enslaves by 3/5 the number of slaves isn't treating them as 3/5 of a person! Morally speaking the number should have been zero (or negative!) if it had to be a question at all.
That’s why you don’t negotiate with terrorists. Sooner or later, they will leverage their position into a better deal at your table, or they will find a better deal at a different table, one which may render any leverage against them moot.
To the degree that increasing that fraction from 3/5 would raise the taxes paid by slave holders, it is not "pro slavery".
Having said that, IIUC, the restriction only applied to "direct taxes" whereas the bulk of US Federal Government revenue at the time was from tariffs and excise, and in any case was a much smaller portion of GDP than today (although to some degree that extends to representation as well, as a limitation of the power being controlled).
My general point stands that the great injustice was all the ways we were treating people as 0/5th of a person, not that we could have taxed some states a little more or given them a bit more power.
