Alternatively, you could say that the point of "legalese" is to define and convey ideas more explicitly than they can be defined and conveyed in English. Try writing a contract in "Plain English" - it will necessarily be full of ambiguity and uncertainty, which defeats the purpose of having a written contract.
Similarly, although programmers don't write code in "Plain English" I don't think we should begrudge them the right to keep using obfuscatory programming languages - even if it keeps the bastards employed.
The argument for specific languages for programmers is stronger because programs are intended to be interpreted by machines. I would be perfectly happy with a legalese that is opaque to the layman if it could be fully parsed and implemented by a machine.
I also believe that, by defining one's terms clearly, it should be possible to write unambiguous plain English contracts. Additionally, an English-parsable legalese needn't necessarily lead to an unambiguous interpretation when parsed as English, so long as it is unambiguous when parsed as legalese, and most of the meaning is retained when parsed as English.