Your opening promises some great design,
And shreds of purple with broad lustre shine
Sewed on your poem. Here in laboured strain
A sacred grove, or fair Diana's fane
Rises to view; there through delicious meads
A murmuring stream its winding water leads;
Here pours the rapid Rhine; the wat'ry bow
There bends its colours, and with pride they glow.
Beauties they are, but beauties out of place;
For though your talent be to paint with grace
A mournful cypress, would you pour its shade
O'er the tempestuous deep, if you were paid
To paint a sailor, midst the winds and waves,
When on a broken plank his life he saves?
Note that the point is not that that the patches are themselves not good (the last sentence suggests otherwise) but that they are not appropriate in the context. A literary work has purple patches when it takes one form of cloth and sews expensive royal cloth onto it in order to make it seem finer than it is. As Horace says later, why would you start out making an exquisite vase and end with making an unimpressive bowl? The mix of plain and purple creates monsters, chimeras.
Very often, however, the phrase is used to suggest that no prose should be purple. This is as absurd and stupid as saying that, because some people sew fine cloth on cheap cloth to make it seem flashier than it is, no one should make fine cloth at all. Some prose is born to the purple, and it is quite as absurd to try to dress that prose in hessian than it is to try to make a patchwork quilt of silk and burlap. But some will say: purple prose is obscure! Yes, that is the point: true Tyrian purple is dark, so dark that it seems black in the distance; but it shines with rich hues when looked at closely in the right light. To the facile glance it seems merely obscure; on closer examination it is imperial richness itself, full of depth and color. That people may look with a facile glance is no reason to write only for facile glances.
We have, then, a Scylla and a Charybdis: elimination of all purple and the abuse of purple. Somewhere between the two extremes there is room for imperial prose.