At the expense of being flamed, I'd say that every back-end engineer can do front-end work even if the end result is terrible from a user experience perspective. You may even find cases in which a person can be equally adept at both.
That being said, I believe that there are more front-end specialists and designers who cannot do what it takes to build an efficient back-end tool.
Try showing your latest application around an office. Few people will comment on how you are grabbing the data, but everyone and their mother will tell you how to make it prettier.
There are also back-end specialists who cannot do what it takes to build an efficient back-end tool :)
The skills involved in web development and software engineering are so varied that I'd hesitate to define too few buckets to categorize people. Traditionally I think the reason designers get away with learning a bit of JS is because of the simple request, pageview and DOM semantic of the web makes it easy to hack on things until they work without the danger of really screwing things up too bad for 3 reasons: 1) you don't worry about persistence, 2) you don't worry about scalability, and 3) any problems you created only live as long as the pageview does.
Newer APIs like client-side storage and pushstate are allowing browser js to quickly evolve into the realm of serious software engineering with even a few twists of its own (like no easy production logs.) Of course a web designer can still come and do a tutorial and duct tape some things together, but this is equally true of PHP, rails or node. There is an inherent learning curve in learning how to operate at various levels of abstraction well enough to engineer a system that meets latency, scalability, correctness and maintainability requirements for a non-trivial app.
That being said, I believe that there are more front-end specialists and designers who cannot do what it takes to build an efficient back-end tool.
Try showing your latest application around an office. Few people will comment on how you are grabbing the data, but everyone and their mother will tell you how to make it prettier.