Hello,
No worries, you'Re not making anyone feel bad. Adding weight there does indeed have disadvantages. It does have the advantage that no matter how far the wheel moves, it will never hit the rack, though, so it does make some sense as a 3rd party add-on. "Beam" type racks don't work well on bikes with suspension, because while they may look fine at first, they are likely to hit your wheel once you go over a bump, etc. - Topeak's design eliminates that worry. You are of course correct about the unsprung weight, but bear in mind that, I personally would basically only use this on-road or for trail riding, not any crazy mountain biking with jumps. (I am also not putting too much heavy in there). I have also examined that Thule have a similar design to Topeak's, which is actually cheaper. (I don't know how the quality compares, since I don't have both).
The Riese & Muller idea looks spiffy, but they aren't selling it as a 33rd party kit, so it isn't useful to me at the moment. This is the kind of thing where 3D printing may make headway in the future!
BTW I have ridden about 60km with this read rack installed and a bag full of maybe 5Kg of stuff without any stability issues, etc. Ine interesting side effect of it being on the chain stays is that it does move relative to the rider, so if you leave the bag sticking up, it will bump into your backside slightly if you do move the suspension a fair amount. This isn't a big deal, and usually if I am on the road I would be riding in lock-out anyway, but I tried it both ways just to see. So far so good anyway.
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