Digital cameras and films to do not have "primaries". The spectral sensitives of digital cameras and films dictate their response to various wavelengths of light. These native responses are sometimes encoded relative to a set of encoding primaries such as rec709, adobeRGB, Kodak ProPhoto (aka RIMM/ROMM), or ACES but these encoding primaries have nothing to do with the camera's ability to sense light of any given wavelength.

Typically the RGB values found in a RAW camera file are those responses dictated by the camera's spectral sensitivities. These are often transformed into CIEXYZ then an encoding RGB space such as those described above based on careful characterization of the camera's response to known spectral stimuli.

The reason that Adobe Camera RAW and Lightroom use RIMM/ROMM as an internal encoding space is due to its nature as a good space for doing tone curve application. See http://www.photo-lovers.org/pdf/color/romm.pdf

If a camera had a set of primaries it would form a gamut in a colorimetric space such as CIE xy, CIE u'v', or CIELAB. That would imply the camera were not able to capture colorimetric values outside those gamut volumes. Clearly this isn't the case. The term "Gamut" is an output device specific term that relates to an output device's ability to reproduce a specific set of colors.

Note : The term "Capture Color Analysis Gamuts" was introduced by Jack Holm, formerly of HP. While the concept Holm introduces in this paper is useful for understanding and comparing camera spectral sensitivities, it is not a "Gamut" in the traditional sense.

http://www.color.org/documents/CaptureColorAnalysisGamuts.pdf