For various reasons, we're often asked for an AI or (more often) an EPS on various projects. The files we provide for nearly all print work (including screen printing and embroidery) is a PDF.

Why? If it's an easy fix, can't you just turn it into X?

Yes, of course. However, files are often being changed right up until the last minute. If every file needs to be saved in various formats, then there is 1) the Illustrator or InDesign CS3 files we're working on; 2) the PDFs we give to the client so that the client can view it; and 3) the X format you need it in*.

*X format is different for every printer, which complicates things further.

Basically, while it is relatively quick to save files in various formats, it does take time (which costs your client — and ours — money) and it introduces a higher chance of errors when we are making several quick changes. If you get a PDF, then you and the client are looking at the exact same file, which minimizes errors and miscommunication.

A PDF is better than an EPS or AI anyway.

It's a cleaner file with more options for editing. Really. If you don't know this already, and don't trust us, then more info can be found here:
http://rwillustrator.blogspot.com/2006/11/whats-in-file.html

At the end of the day, yes, we can provide you with any file format you want. But we want our clients to understand that an EPS is quickly becoming a deprecated file format, a PDF has more options, is oftentimes smaller, can be viewed accurately by anyone, and doesn't have a lot of the baggage of an EPS.