When I was typing it up, I figured it may be subbed on any main, not just the GLA tree. I hadn't thought about a three tier system though.
I'm a believer that if I know a skill, I know it, regardless of my profession. Of course that skill should have a greater affinity when used as a main. Integrating a third tier is just an extension of that, as (in the example) GLA is the main class, PUG is not. I could make that compromise.
The work definitely has to be there, it should take time and effort to gain bonuses to the base skill. Just like meriting did. I'm not sure I'd limit it to just 10 upgrades, either. Probably capping when the third tier effectiveness matches the original base of 20% (13 upgrades in this case)? If the guild cost is multiplied for subsequent bonuses, the diminished return would be naturally prohibitive, unless someone really wanted to invest in it. Starting at a 2k GM base with a multiplier of 1.25 would mean you'd need about 66,500 GM total to reach your tenth upgrade. And over 137k to reach 13 (that's a cost of about 29,100 GM for the 12>13 upgrade of +1%). But when you're at endgame with nothing better to do...why not. If you want a PUG with Soul Eater at full effectiveness, it just adds to the uniqueness as there would be very few people to take it that far, also making that character a little more epic in that regard.
Edit response:
It's somewhat in the game already. But not everything in the game currently is broken either, just needs some tweaking or re-envisioning.