I’m not quite getting where you’re coming from. An Illustrator file contains vector data, which isn’t subject to losing data during compression. Are you referring to raster data that might be included in the Illustrator file or transparent objects that might be inadvertently rasterized or flattened?
I agree, however, that a designer sending an Illustrator file to another designer should just use the native format (possibly zip it if it’s being emailed).
But it’s often not quite so straight-forward.
Logo files, especially, are subject to being passed through many different hands. For example, a designer creates a logo and sends the client several different file formats — .ai, .pdf, .psd, jpg, etc. The client doesn’t have a clue what an .ai file is and can’t open it anyway, so the .ai file eventually gets stored away in some obscure location and forgotten about.
In the mean time, JPEGs and PDFs of the logo, which can be opened by most anyone, begin to multiply and proliferate as people in the company begin to pass them around and make copies. So three years later, when a new designer is needing that .ai file for a new brochure, it’s nowhere to be found. Luckily, the PDF can be found and ends up being the one sent to the designer (after he or she has complained that the JPEG won’t work).