This rule is not so strict, as in my specification, we never pronounce 甲苯（methylbenzene）,乙苯(ethylbenzene), 苯甲酸(acetic acid) in that fashion, though no ambiguity is produced, it is just weird and funny to pronounce so.
However, for familiar words like 奶奶，姐姐, the other extreme is present, which is they are always pronounced as 21-5. (In my opinion, this is an abbreviation of 3rd tones (214) rather than sandhi, as sandhi actually refers to the change of nasal sound between [n] & [m] according to the consonant after it.) This may be due to the level of serenity of words.
For 孔子 and 老子, who are generally regarded 圣贤 and widely respected, it is just improper to do so. (By the way, in the case of 老子, pronouncing it in 21-5 literally means "your father", which is "I" in a very rude way, contrary to 小子, also pronounced 21-5) The only people I have ever know was my professor of classical Chinese, maybe to add some humor to the classroom or just because he himself is "familiar" with them， being old friends in books for him.
两个三声(214)碰在一起合为21-5这一规则并不是强制性的。在化学当中，“甲苯”、“乙苯”、“苯甲酸”这些名词永远也不会用这种方式合韵，这样虽然不会产生歧义，但是怪得很！
但是“姐姐”、“奶奶”这些常用名词就一定要合韵。这种差别可能在于语境的高低。
孔子、老子是古代的圣贤，并且被广泛地尊重，所以用21-5的调子去读很不严肃。（顺带一提，“老子”用21-5来读，字面上变为“你父亲”，实际是“我”的意思，是很没有教养的表达方式，与“小子”相对。）实际上，我长那么大唯一一次听到别人这么说的，是我的古汉语老师。但是他那是为了给课堂增加幽默气氛，并且，老子和孔子对于他而言是在书本当中熟识的老朋友。
