Firstly I disagree with the comment about high speed. The highest velocity bearings are plain: crankshaft bearings in a full size car engine are plain, whereas in a radio controlled model, ball bearings are used. Where velocity (rpm x diameter) is high the oil wedge effect tends to centre the shaft (hydrodynamic lubrication). This can be further enhanced if the oil is pumped.
Straight roller bearings are great at supporting radial loads, but useless at supporting axial loads. Where there is no axial load (for example a generator driven by an engine, either directly through a sliding spline or via belt drive) they are a good solution. In most bicycle applications, axial forces are present, therefore a total of 3 bearings must be considered: roller, plus axial thrust in each of two directions. (in some applications you might find a plain bearing for small axial thrust, but I can't think of one on a bike.)
Ball bearings are much simpler to make, and two ball bearings forms a great all-purpose joint. On a bike wheel, the two bearings are at the ends of the axle, as far apart as possible, which is exactly where we need them to resist non-axial torque applied to the wheel. In this respect, the central part of a roller bearing would be doing nothing, so you might want two roller bearings, one at each end, plus some additional bearings for the axial thrust (getting complicated isn't it?) The better solution is to have a single ball bearing at each end of the axle, with a conical form, which enables it to resist both radial and axial forces. Tapered roller bearings do exist, but they need a cage to help guide the rollers, which avoids the need for four bearings on a wheel, but adds other complexity.
Finally, headsets definitely do need some kind of antifriction bearing. There is an enormous nonaxial torque: your weight is trying to push the forks up and forward, except when you apply the brakes, when they will be pushed back. To be able to steer properly, you need a smooth movement there. Anyone who has tried to ride a bike with a damaged headset will know that it is possible, but uncomfortable and dangerous.