Printer needs output as an .ai (.pdf won't work): please help me explain

Question

Hello, any help would be greatly appreciated.
I have this client who is a great client but for various reasons their print vendors for packaging are incredibly special. I've begged them to change vendors, but it's not going to happen. Their paper printing vendors are fine (great actually), and the packaging vendors I use with other clients are fine, I think it's just because they are using the absolute cheapest vendors.
The situation is that I use InDesign to create packaging (boxes, plastic tubes, labels, the whole bit). It's much more efficient for me because for their specific stuff, I use master pages, character/paragraph styles, update linked graphics, etc. This sort of thing is possible in Illustrator via other tools, but I strongly feel InDesign is a much better solution. When I'm done, I export a CMYK PDF (sometimes spot, but either either way, it's a clean, tidy PDF). I'm pretty comfortable with my settings for this as I don't have trouble with any other vendor.
The problem is that my client is getting emails like this: Is it possible for you to send me the true vector files in either .ai or .eps format? 
I could take the PDF into Illustrator and outline the fonts, and give them one huge, extremely messy package of AI files (and I'm sure they're on Illustrator 8 or something), but the client will need changes after the proof, and there are a lot of files here, so I have to do this, and redo it, etc. Not to mention this client is WAY over budget.
So, what I'm begging for help with: I've been trying to google for something that explains why a PDF is a better file than an AI file for this purpose and why they should be able to use PDF, but I'm coming up with very little. So far I have general Adobe and Wikipedia articles (which covers raster vs vector), but I haven't found any info from a third party that explains why printers should be using (or at least able to use) a PDF to generate plates (poly or otherwise).
Any help would be greatly appreciated. Thanks much.

Answer 1

I actually hadn't thought of that (actually, hardly ever think about .eps anymore). It's a good point though.
In CS4 (don't remember previous versions), I can export these just as easily as a PDF, but then I still have to outline the fonts, zip up several files instead of just sending a PDF, etc. It's 10 minutes, if that; but this is happening consistently with their packaging vendors. I'd rather educate their vendors than do work-arounds for them. I'm not trying to be a jerk about it, but if they don't know a PDF can be just as "true" a vector as an AI/EPS, the problem isn't just outdated equipment, it's a lack of seriously basic pre-press knowledge.
I was sure adobe (or someone) had a nice article on why a PDF was a better solution, but I'm having a hard time finding it. More than a work-around, I'm really looking for general information for them as to what a PDF is (versus AI/EPS). Then I can at least tell my client that their printer requires me to do extra work, and I'll just bill out for my time. It's a nominal fee, because these work-arounds aren't that elaborate, but sooner or later my client will do the math and realize that it's in their best interest to find a better printer. I know a lot of great printers right now that are hurting (although sadly, none of them do packaging), I'd much rather see work go to printers who make an effort to stay educated about their industry.