If my input is of any use:
We all use std::chrono for our internal timing, but it's not far off from sf::Clock.
Sort of like:
CurrFrameStart = std::chrono::high_resolution_clock::now();
TickDelta = std::chrono::duration<double, std::milli>(CurrFrameStart - LastFrameStart).count();
I'd argue that SFML's timing libraries are easier to use, and certainly less verbose, but I see no reason SFML couldn't wrap the C++11 implementation. It would allow, like Laurent said, conversion between representations, and you'd have access to the system's highest resolution clock if needed.
It would also strip off some more platform-dependent code (and remove that oh-so-beautiful "isWindowsXpOrOlder" check), at least for compilers that support the C++11 standard chrono addition (though I've yet to encounter one that doesn't).