Well, 50 pages is incorrect. I realize you posted "50+", but 51, 53, 54, 55, 57, all won't work. A "booklet" must have page counts divisible by 4 at a minimum (8 and 16 are preferred for larger books). So 48, 52, 56, 60 pages, but 50 doesn't work. You may know this.... but I can't tell from here :)
Of course if you are using some binding other than saddle stitching, such as coil or perfect binding, you can get away with 2 page sets.
If the file was provided as a viable PDF then there's little general reason one would need to outline text, especially in a booklet. The only reasons I can think of are...
- The print provider is not using a PDF workflow.
- There's some sort of personalization in the piece.
- The PDF you supplied was incorrectly formatted.
- The embedded AI and PS files need correction (color, trapping, etc)
- The print provider needs to edit something but doesn't want to be held responsible for any typographical mistake which may occur.
- (A common one) You incorrectly used a rich a black for text and they need to correct that.
- In short.... The PDF needs editing, they do not have the font(s) you used, don't want to purchase the font(s), and are aware that legally you can't merely share the font(s) in most instances.
- The print provider has no clue what they are doing.
You are not out of line to ask the print provider why. If they can't or won't answer that.. find a new provider if possible.
They may legitimately have a reason, even if it's "You sent us a bad file" - which they may be hesitant to state outright - happy customers and all that. I generally approach things by asking "Was there an issue with the file I sent? Is there something I can do on my end, short of outlining text on 56 pages, to make it easier?"
I, personally, would strongly push-back if I were asked to outline type on a multi-page piece. It would be non-sensical to me. Heck, I've pushed back on a 4 page piece only to find out it was due to the personalization process - which is an easy thing for me to adjust on my end without outlining anything.
In my office, the provider would be pushed to tell me what needs addressing and why they want type outlined and I'll correct whatever issue it is on my end.
It's important to also realize that print providers deal with all sorts of customers, some more "production aware" than others. They get files in all shapes and sizes with all sorts of odd, uncontrollable, illogical set ups. They may have a standard "script" of sorts which gets them what they need without making the customer feel as though they've done something wrong.
With this in mind, asking "Random Customer" to outline type is a whole lot easier than explaining that they need to trap the artwork on pages 4, 10, and 15. Because then, they might need to explain what trapping is. Which can be a whole other long, drawn out, discussion they'd rather not deal with.
If you understand print production and are capable of editing things which may need editing, then expressing that to the print provider can go a long way to a more seamless workflow.