The major issue with Weather+ is the fact that interlaced video does not compress very efficiently to begin with, and when a subchannel such as Weather+ is deployed, it results in the compression artifacts becoming much more visible.
I'm doubtful there is any bitrate for Weather+ that discriminating HD viewers are going to find acceptable for watching HD sports. The only subchannel that wouldn't have an impact on HD picture quality would be relatively static radar weather maps or low-resolution traffic cams. Anything with any significant amount of motion is going to torpedo HD quality when the primary channel is 1080i.
This is why PBS, which originates its national HD feed in 1080i, allows its local affilliates to convert the HD feed to 720p in order to make more efficient use of the 6 mHz bandwidth available. Still, PBS stations manage to massacre picture quality by deploying too many 480i subchannels -- one would be acceptable, two would be marginal and anything beyond that is a disaster, PQ-wise.
Broadcasters, in general, fought tooth and nail to preserve interlaced video when the HD specification was being developed, and now they are finding just how unsuitable it is for the digital era and multicasting. These guys are dinosaurs, and these are the type of business decisions they will make until they draw their final breath.