On the contrary! I myself have done a great deal of work on a proxy for mobile devices, for a household-name Client. The client software makes certain assumptions of the proxy that would not be valid on the Web at large. But the backend *is* the web at large.
But then the client is either using non-standard HTML forms or non-standard HTTP, neither of which is our concern. It doesn't make any sense to code a general proxy that assumes all chunked requests are meant to be length-delimited just because someone might write themselves a custom client that sends everything chunked. Those people can write their own proxies (or at least configure them to be sub-optimal).
....Roy