I use illustrator a lot and I'm always exporting PDFs. Since the PDF is a "save as" rather than an export, I end up editing the PDF and saving out. This causes version conflicts. Could I just work in PDF? What would be lost if I never used the ai format?
Can you? Sure you can. Should you? Probably not, but it depends a great deal on the artwork and what version of PDF you save to.
You may find when saving to PDF that more advanced appearance settings may get flattened. PDFs don't link to assets, they embed everything - so placed images would be embedded at all times. You may also find that type objects may get converted to outlines. Whether or not these issues affect your work, only you can decide.
Here's the thing, if you save an Illustrator file with the default settings in tact (PDF compatibility on), you actually save 2 files - the .ai and a .pdf in the same container. You can simply open .ai files in Acrobat. If you save a PDF with "Maintain Illustrator Editing Capabilities" checked, you essentially reverse the priority of the two file types and set them to .pdf then .ai. So you still have two file types but now in a .pdf container.
The issue with the two file types is that should there be an issue with the file, the first thing the app (AI or Acrobat) will do to try and repair a file is delete any non-priority file information. So, for example, say you save everything as a .pdf. Then for some reason a file gets damaged. You try and open it in Illustrator, you may end up only getting the PDF data and losing all the native AI data since PDF data is set as the primary data. Again, whether or not this is an issue, only you can determine based upon your artwork.
I, personally, would never trust the PDF format to retain everything I do in Illustrator. Not to mention the fact that linked assets don't exist in a PDF, everything is embedded.
Quite honestly better file management will help a great deal. Similar to how you would have a layered Photoshop file then send clients jpgs, you would never edit the jpg. Using Illustrator in this manner helps a great deal. Always go back to the native AI file. Any PDFs saved would be for client delivery only. So once sent, you could file those away or even simply trash the pdfs and retain only the Illustrator file.
While the PDF format has come a long way, it was never intended as an editing format so it will often change objects to ensure proper output rather than easy edit construction. If you want to ensure you always have fully editable files, you really should save to the native application format.