More concisely, closed-source non-commercial emulators are a prime example of the free-rider problem, which tl;drs down to "they get to use the free guys' research for free, and we don't get to use theirs".

At this point, I think the advantages of open source for non-commercial emulation are obvious and nearly overwhelming. I bag on PCSX2 a lot for seemingly only caring about JRPGs, but it does do an impressive job emulating hardware of near-Saturn-level complexity with order of magnitude higher clocks. And Dolphin's been even more impressive, even if I think they're well into the gray area with Wii support. Outside of the Sega space[1], open source rules the roost.

[1] Which is super odd. Genesis, Sega CD, 32X, Saturn, Dreamcast, and Model 2 all have only closed-source reference emulators, although I think more MAMEdevs have ElSemi's Model 2 source than have up to date MAME trees.