Any time you claim to be a professional X, you're staking your reputation on a claim to have specific, comprehensive expertise in that profession. With typography, some graphic designers can justify such a claim, but most can't stretch that far.
"Typographer" implies professional expert. Most good designers are adept at typography, and many be typography enthusiasts, but only a minority have gained that specific, comprehensive expertise:
Expertise: Could you do the things professional typographers do? Could you do it all day, every day? For example, imagine you work for a publishing house who are thinking of developing a new unique typeface for one of their publications, like The Guardian did under designer/typographer/creative director Mark Porter. Could you:
- research and make a coherent case for or against there being a business need for this from a typographic point of view?
- identify what typographic properties and features the new typeface needs?
- choose the most appropriate foundry and/or type designer, and explain clearly and coherently why?
- oversee the typeface creation process, knowing when to demand, for example, higher or lower contrast or x-height (and why), which optional glyphs to include, what weights and variants, which languages to support?
- produce or oversee the production of guidelines on correct usage of each variant?
Specific expertise, deeper than that necessary for graphic design. You'd be expected to know the finer points of typography even if they don't often come up in the context of graphic design, from the advanced things OpenType is capable of to the difference between a "point" and a "pica". No-one knows everything, but a "typographer" will love the opportunity to enthusiastically geek out about the fine detail side of typography even when they're not necessary for an active project. A few semi-random examples of topics I'd expect typographers to have opinions on but average designers to struggle with:
- Appropriate uses of discretionary ligatures
- The pros and cons of hinting
- Typography trends such as foundaries like Tart Workshop who use high-tech OpenType features to simulate low-tech hand-lettering
- Anything to do with the history of typography, movable type, letraset...
- Typography in-jokes, like, what's the most embarassing
keming you've seen?
Comprehensive expertise: Do you have the breadth of knowledge of a typographer, or do you only know those parts that touch on graphic design? Can you give genuine, informed, professional-level advice on, for example:
- typography for paperback novels?
- typography for low-quality paper?
- typography for signage and wayfinding?
- typography in cartography?
- international typography and scripts? You don't need to be a linguist, but I would expect very basic passing awareness of what issues exists - for example, the importance of character encodings, or the fact that some scripts like Arabic are unreadable without correctly-applied ligature-like joins.
Most graphic designers (myself included) don't have this level of expertise, and can accurately describe themselves as designers and typography enthusiasts - but to be a on a par with a professional typographer requires an extra level of expertise.
Some designers have this extra level, some are close or working towards it, but most don't.