Indeed... But very bad Idea :D
In Canvas 2D, the spritefont plugin would draw each tile one by one (almost as bad as using tonnes of sprite directly)
Whereas you usually want to use context.createPattern() to make something tile (that's what the tiledbg plugin uses)
In WebGL the spritefont plugin would draw a quad for each repetition (faster than canvas2D, but still).
Whereas you would probably just wrap a texture on a big quad.
Knowing that the major bottleneck in html5 is the rendering... bad idea =)
And in both cases, using createPattern (canvas2D) or a big texture (WebGL) you would avoid seaming issues.
Personnally, I made a quick test on JSBin and animating a tiled bg using image frames should be possible.
http://jsbin.com/aqowin/2 (tested with canvas2D)
I'm not sure though how much strains it puts on performances. But any cases, redrawing big chunks of screen (and tilebg are more likely to be big objects) is expansive.
I'm not entirely sure why tiled background aren't animatable, but I would bet on the "big todolist" syndrom =)
And well... you can always emulate animated tiledbg by using many objects and alternating their visibility. (I did that once)