While profession is typically used in C&G for income gained, labor points aren't really applicable to a magus's personal servant. Are there any other benefits to a servant having a high rank in this Ability?
Your master's clothes, hair and beard will always be immaculate, and his food will never be too cold.
In a castle or manor, Profession: Servant can be used as the equivalent of Stealth, because you know your way around the servants' passages.
You know when to prudently be somewhere else when your Flambeau master is in a rage.
...More seriously, there really isn't much of a mechanical reward for having a high Profession: Servant, because "domestic servant" is kind of a dead-end profession. At best, it'll get you appointment as a custos, in charge of managing the other servants in a particular area.
The rules in City and Guild are specifically designed for craftsmen and merchants, and do not generalize well to peasants, clergy, nobility, or (ironically) laborers.
"servant" meant many different things back then. It was often a very high status calling, and a respectable profession even for nobles, in service to someone of higher status, of course. Body servants or personal servants were generally higher status than generic house staff, and the status of your head of household often determined status of the servants, as well.
Indeed. He mentioned a covenant servant, however, which means he lacks a lot of the opportunities that a young nobleman on the butler's staff has. The benefits of high skill mostly accrue to the master, and the benefits that such a talented servant gets are dependent on the master's largesse.
For actual benefit accrued, I would suggest the primary benefits are roleplaying hooks, such as being able to circulate in a more refined crowd. For mechanical benefits, I would suggest using the quality metrics, i.e. a skill of 12 grants a +1 in a single applicable situation, a 15 or higher grants an increasing bonus with a wider effect. A lenient storyteller might allow a comb of Quality or a magic footbath to enhance this mechanic.
Nothing "lenient" about that! Getting a bonus to shave someone with an appropriate razor of Quality is the kind of thing that Items of Quality are designed for.