I don't think Mark was thinking about outsiders when he started this, and each generation of AMD gpus has been somewhat evolutionary (at least up to the GCN cards), so I understand his starting with the R600s and working up, since the last adopted series was the R500s.
Occasionally he'll drop a little info on me that gives me some insight into the changes, and its been pretty cool.
The R600s and R700s are extremely closely linked, but the jump to the 5000 startled me in how much was gained (even lower end card like the 5650 and 5750 are a better choice than the 4870).
So yeah, there more than a little utility for us with these cards.

A system with an HD 5750 is a quantum leap over the X1900XT.
And the jumps up from there to the 6750 (with is Juniper Pro gpu, just like the 5750) or the 7670 (which is a Turks XT gpu and actually a little lower in performance) aren't that impressive.

And still, I really like the 6670, 6750, and 6770.
They perform as well as any card in the 7000 series below the 77XX.

Of course, all of these are Terascale cards.
GCN doesn't start until the 7730.

And how much will we get out of GCN? I doubt it will be much. The operating frequencies are only slightly higher, we have no Gen3 PCI-E slots (so that feature won't matter), and our drivers uses a surprisingly small number of graphic primitives (think about it, what executes on our highest end cards also works on a Radeon 9200 - or even lower cards).
No, what's important for us are transfer rates, basic copies to and from memory, fill rates, primitive functions that are improving, but don't reflect all the new functionality built into a modern gpu.
In short, I saw great leaps in DX12 performance with the GCN cards, but in DX11 performance sometimes the 6000 series cards did better than the 7000 series cards.

So, with Linux being a consideration, I'm probably sticking with a 6000 series card, either the 6670 or the 6750.
Believe me, these will be scary fast under MorphOS.

Heck, even the R700s will have something to offer, because they are much faster than the R500s and the driver will offer composting (so far this is the highest gpu that that function will be supported for).

Which really makes me hope Mark does give us AGP R600 and R700 support.
A G4 or G5 with a Radeon 3850, 4650 or 4670 would offer compositing AND be much faster than our current cards, I don't doubt for a second that a well written MorphOS driver for these cards will easily allow our G4 systems to step all over Tabor (let alone what a G5 could do).

So YES, there ARE reasons to support cards below the HD 7750.

We just need to stop thinking in terms of OS4, after all, Linux users of A-eon and Acube systems use cards below the GCN series too.