Actually it is routinely being done, but only if no such component is available or they are not practical. The design and characterization requires time, which could be saved, or reduced if an off-the-shelf component would have been used. SMD capacitors with values smaller than 1pF are available.
You can always create a small capacitance value with series connection of multiple capacitors if the series resistance and inductance does not matter. That could be also a way to go.
In RF design, matching networks are used to avoid reflections and get out the maximum power from a given device. The simplest case, where I've used a trace capacitance was when I needed to match a bondwire chip connection to a transmission line of the PCB.
In a PCB --- or in any lithographic process --- the trace width is not well controlled. The thinner the copper and the higher the resolution of the technology is, the better. The substrate thickness is fairly well controlled, especially in substrates intended for RF applications. I have no information about FR4, but you can ask such data from your manufacturer. Therefore I would use the capacitance between metal layers, rather than the capacitance between traces of the same metal layer. Especially if you have a ground plate there, its easy to do. If the capacitance value is critical you might run some EM simulation, or parasitic extraction tool, but if you have no licence for such tools, a test PCB run and measurement might do the trick. Since the trace width inaccuracy will alter the plate area of your capacitor a circular cap would fit the best.