There are only a few interesting major Hermetic virtues. Core has ten such virtues, and two are well known to be overrated. I would not be surprised to see slightly more than one magus in eight take Mercurian.
Not having a major Hermetic virtue would be unusual I think. It is a major spice to add flavour to a character.
On top of that, there is a canon Mystery Cult known to initiate Mercurian.
Even in long running and fairly high powered games they tend to be a pain for characters without the combination of Virtues. Similar to your experience, my groups current game has not one but two characters who have created several of the rituals. The difference is our game is over 100 in game years and none of the starting characters have Mercurian Magic.
Even with all that time and a good Vis income, none of the Magi are close to +5 in most Characteristics. My Magus has +5 in Int and Com, but he started with +4 and +3 in them. The newest Magus actually does have Mercurian Magic, but he is the character of the primary SG and closer to a PC/NPC relationship with the rest of us than a PC/PC relationship.
See that's how I see it, all flaws can be circumvented or fixed... essentially, over time. Your weak characteristic flaw has become a Vis Poor flaw, because you have spent all your Vis and study time on finding someone to and paying to fix your physical flaws/ social drawbacks. While everyone else in the troop has gained the 100+ pawns of vis, you spent... Therefore I see no issue with this, after all the Illiterate can be taught to read, the duty bound can pledge to new mastery and the poor can (eventually) gain wealth.
For my home game, we rules that the virtue for Poor Characteristic was essential and not fixable.
As for powergamey and casting those rituals, it all depends on how long the game is planned to run. My weekly game has run for a few years of real time, and we only bothered to learn Pre and Com +3 boosting rituals because we lost interest.
I have rarely if ever seen the single target versions cast. I have sometimes seen them learned, and then immediately re-invented as Circle target. Suddenly, each casting is helping 10-50 people. That is when things start getting... Strange.