But when defining a gaming system in terms like Thief or Fighter, the system needs to maintain a balance, and in this case Mormegil has identified a valid weakness in the system.
There is no such thing as "thief" or "fighter" to many people. Many people play characters, and they pick the profession which best supports the skill set for that character.
A Fighter has no additional favorite categories compared to a thief, and the professional abilities are 3, each worth 10 DP as individual talents. If you are a thief there is no reason to currently take on a multi-profession fighter. You would spend 20 DP (or 15) for access to fewer categories at 2DP and gaining a 10DP talent. Why bother?
If you are creating a new character there is no valid reason to take a Fighter based upon the mechanics.
Take the Thief profession and get access to additional categories to allow expanded flexibility in the future and if you really want the heightened skill specialization (weapon), Lightning Reflexes and Shield Talent - pick them up as talents for 30DP out of your starting 70-80. The only reason not to do this, is if you have no intention of adding any subterfuge skills to your character, where the thief gets 6 of them. Of course you could put those ranks in Ambush, Sniping, and/or Streetwise and still have a very "fighter-esque" character.
The lack of a powerful Professional Ability for Fighter makes them a weak choice as a second profession (especially as a compliment to a thief or rogue character where the favorite categories are complete overlaps).
The only type of character that a Fighter profession truly benefits would be a tank or fully focused combatant, and that benefit is minimal (only about a 15 DP differential if your calculate it out).
So - Mormegil, thanks for pointing that out. I'm sure it has probably been mentioned before since HARP has been "House Ruled" by many gamers on these boards, but if it has, it has been a while.
Still, I don't believe the multi-profession rules are broken - I believe the Fighter, Thief, Rogue professions need another look to address their overlap.
BTW - No one in their right mind would ever add a rogue profession as a second profession due to the lack of any kind of professional ability.
IMO
Thieves should not have Combat as Favored
Fighter should have a more powerful Professional Ability in place of 2 of the 3 abilities listed.
Rogues need a review in terms of professional abilities, and they have favored categories in almost all areas.
Of course this now opens pandora's box and you need to re-evaluate all of the balancing between professions.
Not every profession is going to be "numerically" balanced with the others, and some may only work better for role playing purposes, but the one issue you pointed out is significantly a concern to me. Thanks!