It is not useless and can, indeed, be logged. Typically, FOs will fly the empty Part 91 legs, and log 61.51 PIC. Live legs, the SIC is required by 135 or 91K and thus SIC is logged.
Depending on your career goals, there are definitely better stepping stone jobs out there, but all in all it is a decent company.
Back in 2011-2012 upgrade was around a year, I don't know what it is right now. They seem to be gaining a net 2 or 3 airplanes a year, so upgrade is based on growth and attrition. That said, upgrade is also merit/performance based, not seniority. You will need ATP minimums, and a few letters of recommendation from captains you have flown with.