Moreover, you only get ONE of the benefits of the new profession.
Note that this has important implications for planning a character's progression.
e.g. if you plan to make a magic-using rogue type, if you started as a Rogue and later added the Mage profession, you'd only get to choose
one of the Mage's
Sense Magic, and
Mage Sphere abilities.
But if you start as a Mage and add Rogue, the Rogue only has one professional ability (+5 to a favoured skill every 3rd level) so that's what you get.
Similarly, a Thief has two professional abilities,
Subtle and a +10 bonus to any Subterfuge skill every 5th level. If you add the Thief profession, you have to choose
one of them. You're better off starting as Thief and adding Rogue than vice versa.
Sometimes it's just better, more versatile, to just "buy" the professional talents you want. e.g.
Lightning Reflexes and
Shield Training cost 10 DP each - unless you need the Combat or Athletics categories as favoured, you'd be better off just buying those Talents for your character than adding Fighter as a profession (also 20DP,. giving you just one of those talents).
It's often better to just buy an
Arcane Power (30 DP) for a Rogue (or other prof that already has Mystical Arts as favoured) to get a spell sphere than it is to buy
Additional Profession (20DP) - it costs more, but you can increase your spell skills at every level, not just when you're advancing a Mage or whatever level. And some GMs will interpret a human's
Professional Adaptability very broadly, applying the 5 DP discount (or even 25%) to
Arcane Power,
Arcane Circle, and similar Talents, as well as to
Additional Profession.
Finally, Rogue makes a great
Additional Profession to "round out" any other character: in addition to the +5 bonus per 3 levels, the Rogue has
lots of Favoured categories so is very useful for adding some Athletic, Combat, Outdoor, Mystical Arts, and/or Subterfuge skills to any other profession. At 20 DP, it's more than worth it if you plan to buy more than 9 skill ranks in any non-favoured skills. e.g. 10 x 4DP = 40 DP total. 20 DP + 10 x 2 DP = 40 DP. It's even better for human characters, the break-even point is around 8 ranks (8x2+15 = 31 DP vs 8x4=32). Given that even a Mage is likely to want to be able to full-parry with their quarterstaff in an emergency, it quickly pays for itself
Also, the +5 bonus you get to a favoured skill is
at least equal to a free skill rank, so the break-even is effectively 9 ranks for non-humans, 7 ranks for humans.