Can a character use its secondary professions weapon to full effect with its skills, etc?
eg. Warrior using a bow
Dervish using daggers
Using secondary profession weapons
Pit Bryan
wolfmane
Yes, as long as the stats meet the requirement of the weapon 
halbatross
Yes you can, but your damage potential is lower if the weapon attribute belongs to your secondary proffesion. This is because you can only apply runes of your primary proffesion to your armor.
A D/W for example can only have up to 12 Swordsmanship (which would always cost 96 attribute points), but can have up to 16 Scythe Mastery. The result is more potential damage and Critical Hits with your primary weapon (scythe). Also, going for less than 16 Scythe Mastery, with a Scythe Mastery headgear and a rune (even a minor rune), will cost you less attribute points than 12 Swodsmanship, while providing more damage output. You won't have that kind of flexibility with other weapons.
A D/W for example can only have up to 12 Swordsmanship (which would always cost 96 attribute points), but can have up to 16 Scythe Mastery. The result is more potential damage and Critical Hits with your primary weapon (scythe). Also, going for less than 16 Scythe Mastery, with a Scythe Mastery headgear and a rune (even a minor rune), will cost you less attribute points than 12 Swodsmanship, while providing more damage output. You won't have that kind of flexibility with other weapons.