Perfect replicas of the 40k models would always be prefereable. However public visibility and availability means that people, who are not aware of these forums and/or would prefer to trade with a tracable company, have a viable proxy option. (Not implying that any of our local "suppliers" are in any way dishonest, but as an outsider you would't know that for certain). All I'm saying is that a "Skunk works" production would always tend to be more reserved for an inner circle. Proxies that are easily available (and with regards to the Skeletons wouldn't be modified any more than what the sloppy casting in the epic Character packs results in - ie. the blade under the Skeleton rifle missing, and that sort of thing) are quite simply more likely to get out to a lot of people.
Tyranids are tricky, but the skeletons would be so close to the original that I seriously doubt anyone but the top 5% of painters on this forum would be able to paint the un-modified skeletons to look any more Necron-ish than the proxy (Now there's a chalenge for an EpiComp painting competition right there
). As I mentioned in the Wargames Factory Android thread people put way too much into a name without actually looking at the model itself. This can be used to your advantage (as DRM did with their Andrayada - They were painted silver and called robots, but they don't look Necronish at all. People could have used them as Eldar or Space Marine proxies just as well)
With all respect to all the great CAD work being done right now but I think people are overexaggerating the importance of details compared to what can actually be picked out by your average painter. If we take the FW grey knights as an example, I've only seen a few that were actually painted well enough to really bring out the detail on those models. The FW Leman Russ tanks looks great but they are a royal pain in the posterior to shade and highlight properly with any other technique than washes and drybrushing, making camo paint jobs that aren't simply monochrome quite challenging. It is quite interesting to note how the fashion in painting styles and techniques change as models that invites to certain techniques become popular.
Anyway, I digress
The point I was trying to make was simply that Skeletons with guns would be no less Necronish than the old Chaos Androids people have been quite happy with thus far. If I am then given the choice, I'd rather see them freely avaliable to everybody rather than a select few who frequent these forums
Rant over