I don't have any experience with the Canon 35mm f/1.4 L, but I think the answer to your question is probably "it depends."
What do you have in mind when you say "portraits?" Do you mean tightly framed head shots, or do you mean environmental portraits that give context to your subject? A 35mm lens may not be "perfect" for either of these, but it may be "pretty good" at both. Indeed, I would submit that 35mm is a fairly versatile focal length on a crop-sensor body, especially if you are going to limit yourself to a single focal length.
I would be careful to avoid confusing "distortion" with "perspective" (or "exaggerated perspective"). Distortion is a function of the optics of a particular lens, and might manifest itself as barrel, pincushion, or complex distortion. Perspective, on the other hand, is a function of the distance between the camera and the subject. All else being equal, as you decrease the focal length (i.e., move to a wider angle), you have to decrease your distance to the subject in order to maintain [EDIT: similar framing]. So if you are able to fill the frame at, say, 6' on a 50mm lens, you may have to move, say, 2' closer to fill the frame with a 35mm lens. Because you are closer, you will see more exaggerated perspective (for example, an exaggerated nose) with the 35mm lens, all else being equal.
But all else isn't always equal. Many people consider 50mm to be an acceptable focal length for portraits on a full frame camera. (You may, of course, disagree with that premise, but we have to start somewhere.) A 35mm lens on your crop-sensor body will provide about the same field of view at a given distance and should therefore show similar perspective to the 50mm/FF combo. And if you want to be precise, the 35mm would actually have a slightly narrower field of view, so you would probably have to slightly increase your distance to the subject, thereby resulting in less exaggerated perspective, when compared to the 50mm/FF combo.
So to return to your question, can you shoot portraits with a 35mm on a 70D? Sure. How will they compare to portraits shot with a 24-70mm f/2.8? Well, if they are both at 35mm, they'll be reasonably similar, all else being equal (aperture, etc.). You may see differences in lens-based distortion and bokeh (the quality of the out-of-focus portions of the image, not to be confused with the depth of field), and there may be a number of other tradeoffs (prime vs. zoom, max aperture, IQ, size/weight, cost, etc.), but the perspective should be the same.
Which isn't to say that a 35mm would be my lens of choice for tightly-cropped head shots. But I wouldn't be so quick to dismiss it, especially if you want to shoot group shots with the same lens.
Just my opinion of course.
