For half or quarter step supersampling it's not always the case, depending on the source image. High frequency areas can show extra aliasing if the scalar is rubbish. You can see it with a simple test yourself in Photoshop. Create a 1920x1080 image with white background and then raster some large black text and a couple of black tris. Then resize to 1280x720 with nearest neighbour filtering (point sampling).
Why would anyone use nearest-neighbour sampling and discard half their rendered information? Downsampling I'd expect at least a straight bilinear that helps reduce aliasing. Take your Photoshop example (doesn't need a whole 1080p screen. Just downsize to 66% of whatever size original you start with) and use bilinear sampling, and the results are better than a straight 720p render.
In fact I'll do it for you, seeing as I'm so generous
'1080p' source vector graphics
'720p' source vector graphics :
Downsampled 1080p using point sampling :
Downsample 1080 using bilinear sampling :
The pointwise downsampling is horrific, and I can't imagine anyone, whether on the TV or the game output, would use it. I know the earliest LCD monitors did, but no-one still uses point-sampling, do they? In contrast the bilin sampling is a marked improvement over rendering to 720p without AA. Given a choice between 720p no AA, or 1080p no AA downsampled, I know which I'd choose! And yes, instead of rendering to 1080p and downsampling you could render at 720p and get more efficient AA, but as a quick fix to cater for all resolutions downsampling seems an okay tradeoff.
This shows the worth of 1080p downsampled to 720p, but this isn't an indicator of output quality on the final TV which is dependent on screen size. IMO there's no point discussing this on paper. The only way to say whether supporting 1080p is a 'bad thing' is to have two similar TVs, same screen size, one 720 native and the other 1080 native, and render 1080 and 720 screens to the both of them. There's no other way to compare different native fixed-resolution displays, and know if 720p screens upscaled to 1080p is particularly bad or not.