Dunno.... I am kinda torn on Mentors.
I personally don't use them when creating a mage, as I find the hinderances that come with them a little too.... conforming for my tastes.
And as a GM, I have noticed 2 things very consistantly with mentors:
1: player who use Mentors, ALWAYS remember their bonus dice. And ALWAYS forget the penialties.
2: enforcing the penailties from a given mentor usually annoys/upsets the player.
So, yea... too many (IMO) are looking at mentirs and seeing dice, and NOT seeing them as what they are, a balancing act.....
However, I have had SOME players who play up their Mentors masterfully..... so, yea.
The drawback is one of my biggest considerations with a mentor. If I take Dragonslayer as a mentor, I know I'm bound to that level of honesty or I'm gonna tick Dragonslayer off.
That said, there's nothing that necessarily says having a mentor spirit is necessarily voluntary. I've seen someone run a Christian theurge who less-than-voluntarily switched mentors from Dragonslayer (as an aspect of St. George) to the Seducer (as an aspect of... well, you can guess; probably would have been The Adversary had Hard Targets been out) for background reasons, and any time he made those charisma+willpower rolls, that was an active fight against his mentor.
Okay, please get past me saying "True Hermetic" in the OP and focus on the questions I was asking. And yes, I AM a grognard who played 1-3 ed, and would you like to hear how I had to walk to school uphill both ways in the snow? 
As you say, they ARE a good mechanical bonus and the mechanical disadvantage is usually somewhat minor. So let me rephrase my question: How many players do you see take totems Mentor Spirits because they want to explore roleplaying a hermetic mage who filters his view of magic through the lens of a primal spirit and lives by its guidance, vs how many go "Oooh, five karma for bonus dice, shiny!" 
I've never seen anyone take a mentor spirit without it informing their roleplaying.
While yes, many of them pick the totem for mechanical reasons, either the choice rolls back into the character, or the character rolls back into the choice in some way.
I've never seen someone take a totem without making a character who can live with their drawbacks. Those who take Dragonslayer bring characters who
have that honest streak so they can live with those oaths, those who take Bear
have that protective streak so they can live with Mama Bear mode. Those who can't deal with Mama Bear mode don't take Bear.
And the names of the mentors vary from tradition to tradition. Dragonslayer to the Shaman is St. George to the Christian theurge. Chaos to the Chaos Mage is Loki to the Norse mage. The Great Mother to a Wicca may be the Virgin Mary to a Christian theurge, and may be the spirit of your great, great, great, great grandmother watching over you to a Shinto priest. What it is to a hermetic is for you to decide as a hermetic.
Last time I saw a hermetic with a mentor spirit, that hermetic was also Catholic (though not a Christian theurge) and had Fire-bringer (St. Thomas Aquinas) as a mentor spirit.
All that said, hermetics are rationalists. Mentor spirits, at this point in the timeline, are objective truth. A hermetic denying mentor spirits on the grounds of rationalism at this point are like a flat Earth skeptic today.