Quite. Only it's worse than that; ideally we would want a mapping that allows coordinates to be interpolated *linearly*, which is possible for at most half the sphere if your texture is infinitely large.
But if you're willing to live with a bit of distortion you could have a mapping that's at least continuous everywhere except at one point, which I can imagine being useful (and seems to have been the intention behind the current texmap spec.)
Another thought that occurs to me is that if the texture were initially represented as a cube map, it wouldn't be too difficult to convert that on the fly into 6 partially-overlapping planar textures that older rendering hardware / APIs could cope with (thus giving proper interpolation and nearly seamless results so long as the polygons are fairly small - you could even be smart and generate textures that are exactly as large as you need.) This would in fact be a lot easier than converting a cube map to or from some polar-coordinate-based projection. I can see, though, it'd be annoying to deal with the idiosyncracies of cube map coordinates (whichever way we chose to define them) and I can't think of any particularly elegant way to express a cube map in an LDraw extension.
But if you're willing to live with a bit of distortion you could have a mapping that's at least continuous everywhere except at one point, which I can imagine being useful (and seems to have been the intention behind the current texmap spec.)
Another thought that occurs to me is that if the texture were initially represented as a cube map, it wouldn't be too difficult to convert that on the fly into 6 partially-overlapping planar textures that older rendering hardware / APIs could cope with (thus giving proper interpolation and nearly seamless results so long as the polygons are fairly small - you could even be smart and generate textures that are exactly as large as you need.) This would in fact be a lot easier than converting a cube map to or from some polar-coordinate-based projection. I can see, though, it'd be annoying to deal with the idiosyncracies of cube map coordinates (whichever way we chose to define them) and I can't think of any particularly elegant way to express a cube map in an LDraw extension.