It's unprofessional because of the graphical glitches and general distortion caused by viewmodel_fov 90.
The default viewmodel_fov is 54, which is huge but also meant to be paired with fov_desired 75. The equivalent viewmodel_fov for fov_desired 90 is 64.8; a lot of players use 70, which is a close-enough approximation. Graphical glitches do still occur, but they are significantly lessened with ~70 viewmodel_fov.
Casting is obviously different from playing the game, especially if you are an organization focused on making high-quality productions like TeamfortressTV. These productions should have a strong emphasis on aesthetic appeal, and to that end we have some AMAZING quality assets in use, e.g. the TF.TV HUD, overlays, plugins, etc. Which is why I'm upset when all that professional, high-quality content is tarnished by elongated/broken/missing arms--stuff an outsider would probably see as an amateur mistake. You might not think this matters, but in the production industry, quality control is a top priority, because a single "mistake" can make the entire production seem unprofessional.
Imagine a movie where one of the actors wore a green mask to all the green-screen shots, and the production studio chose not to CGI over it, and so in the final movie, he was just without a face. That would be unacceptable; people would be FIRED. TeamFortressTV isn't a "professional" production studio, but we are making a similar mistake by allowing these errors in our productions. Even real, professional studios sometimes mess up, but they try to avoid them as much as possible. I don't see a valid reason not to apply the same principle to our own productions.