You have to understand, though, that legalese isn't English. It may look like English, and even work like English in ways, but its not English. Saying that legalese and English ought to be equivalent because they share words and syntax is like saying C++ and Java ought to be equivalent because they share keywords and syntax.
For patents in particular, though, a big part of the justification is that they constitute a disclosure of the invention to other practitioners in the field. It's a quid pro quo: we grant you a limited term of protection, in return for you divulging the details.
As part of that arrangement, it seems like we ought to require that the disclosure be in English, not merely in a tongue that shares some English words and syntax.
English? Hah. I'm against the whole patent charade but if we're going to grant them, the inventors should be forced to disclose the entire goddamn source code, development infrastructure, and version control repository which proves that they've actually invented something and not just handwaved in the direction of an idea.
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.