Can the CMMI be applied only for software developments companies, or can systems integration companies also derive benefit from it ? What I mean by "systems integration companies" are organizations who engage in selling IT solutions including sofware and hardware as a solution.
The CMMI is not a software only model. By the end of this year, the SEI will have released three CMMI constellations: CMMI for Development, CMMI for Acquisition, and CMMI for Services. The Development and Acquisition constellations are currently available, the Services constellation is due to be released this Fall. The CMMI for Development (CMMI-DEV) covers hardware, software, and systems engineering and so it will apply to systems integration companies as you have defined the term.