Then why would THX recommend 5.5 ft for a 50" at 16:9? THEN, if you ask certain others, 36 degrees is the recommended angle for the BACK row of the theater.
Then what about 2.40 AR? Or even just 1.85? Do you think the majority of films are made at 1.78?
If you are NOT basing viewing on television programming, but on film, then you can increase the recommended viewing angles.
Did you know that many people with anamorphic setups sit at 1:1 ratio? That would mean that with a screen as wide as the 50", but at 2.40 for instance, you would only be 43.59" away.
I have a 159", and I can't detect pixelation until I am about 5 ft away, or closer, but then again JVC PJs are known for super high fill, or perhaps lack of pixelation, which for others could mean "less sharp".
THX bases their recommendation on size for a proper theatrical experience. That is a different issue from whether or not the picture will be perfectly clear (as far as format resolution is concerned, which is not the only relevant factor in practice). You can see for yourself that THX makes no mention at all of resolution when giving their recommendation for distance:
http://www.thx.com/home/setup/display.html
With a 50" TV, they recommend sitting from 5-7.5 feet away (though using their 'divide by .84' method gives a distance of about 4.96 feet). The minimum common resolution that is necessary for a clear picture within their recommended range is 1080. With a 1080p TV, the calculator I suggested gives the optimum distance of 6.5 feet with a 50" screen, which is within the THX recommended range. But if you sat that close to an LCD or plasma TV of that size that was 720p, you would see individual pixels (if you have good vision). In my opinion, being able to see pixels makes it look like crap.
I should, however, have been clearer in my post. With fixed pixel screens, such as LCD and plasma, sitting too close makes the pixels clearly visible. In the case of a front projector, one is not looking directly at pixels, but is looking at a reflection of them, which affects one's ability to distinguish individual pixels. So you can generally get away with sitting closer to a front projector screen than what I have stated above before it looks so very bad, though one will be able to see a "softness" to the picture if one gets closer than the optimum distance that is calculated by what I have recommended (again, assuming 20/20 vision):
http://www.digital-digest.com/articles/viewing_distance.php
Go to a store and get close to an LCD or plasma TV, and you will find that (if your vision is good) if you go much closer than 6.5 feet to a 50" 1080p screen, you will start to see pixels. How much closer will depend upon your vision; the worse a person's vision, the closer they can get before they see particular details. And just try getting as close as THX recommends with a 720p LCD or plasma set, and you will really see why resolution needs to be considered in making a judgement about distance.
See also:
http://www.practical-home-theater-guide.com/Tv-viewing-distance.html
http://www.soundandvisionmag.com/hitech/1137
