There still can be benefits to releasing a 4:3 show on Blu-Ray, especially with older (made before 1999 or so) anime that were animated on cels and stored on film. In those cases, the film can be rescanned at a higher resolution, thus making it "True HD." Depending on the original quality and condition of the film, it might not quite be 1080p-worthy, but chances are it'll look better than DVD. Cardcaptor Sakura, which was done on 35mm film instead of the usual 16mm, is supposedly a good example of this.
Not a bad thing! It makes me wonder why they bothered releasing it on bluray at all. More episodes per disc is the only benefit, as I see it.
Newer shows from the early/mid-00s were generally animated digitally in SD resolutions, which means any BD of them has to be upscaled. But that applies equally to 4:3 and 16:9 shows from that era, aside from some exceptions like Twelve Kingdoms, GitS:SAC, and Samurai 7, which were animated in HD. With a good authoring process that doesn't go overboard on damaging filters, Blu-Ray upscales can offer:
* Better upscaling algorithms than what people's DVD players and HDTVs can do
* Fewer compression artifacts, thanks to more efficient codecs and more storage space
* More storage space for lossless audio
* Better color balance, all progressive video (I think), and other minor video advantages
So the thing to do would be to (A.) take the age of the series into account (B.) see what people say about the quality or upscaled nature of the source, and (C.) DL the highest "worthwhile" resolution.