i've never experienced any problems saving the .ai file as a .pdf. pdf's are nice simply because anyone can open the file - even if they don't have illustrator. the resolution of the backround images should stay the same, and any vectors (text, lines, etc.. ) you have in the file will remain as well.

depends on the settings. By default, Illustrator should zip the bitmaps, not compress them, but I don't know if that's always the case.

I set the compression to jpeg when exporting as PDFs.

I think it's fine to accidently save as a PDF, but I would never keep the file as that. You won't know if the embedded images are compressed or not (it's similar to saving high res images as TIFFS or Targas instead of jpegs - it's just a better way to keep quality at the best level)

I always keep an AI version for maximum editing capabilities.

.ai is illustrator's native format - if you can do it in illustrator, you can save it in .ai

.pdf is adobe's portable document format - it is designed to support a wide range of document data, but not necessarily everything that .ai can store.

when you save in .pdf, illustrator warns that not all features of your document will necessarily come out right. if you save in .pdf, make sure you check the file to see if everything is right.

any resolution of embedded files is supported in both .ai and .pdf

i like working in AI, PDF is for finished stuff so i can fully edit.