Oh because they should switch to gladius for close-combat, but the original Triarii in RTR has no gladius 
The fix is simple. Open RTRPE\Data\export_descr_unit.txt, find the line "type roman triarii":
Code:
type roman triarii
dictionary rtr_roman_triarii
category infantry
class spearmen
voice_type Heavy_1
soldier roman_triarii, 35, 0, 2.0, 0.45
officer roman_early_centurion
officer roman_early_standard
officer roman_aquilifer
mount_effect horse +3, camel +3
attributes sea_faring, very_hardy, can_sap, hide_forest
formation 1.0, 2.0, 4.0, 4.0, 5, square, shield_wall
stat_health 3, 3
stat_pri 13, 1, no, 0, 0, melee, blade, piercing, spear, 0, 0.4
stat_pri_attr no
stat_sec 14, 1, no, 0, 0, melee, blade, piercing, sword, 0, 0.35
stat_sec_attr no
stat_pri_armour 10, 11, 6, metal
stat_sec_armour 0, 1, flesh
stat_heat 4
stat_ground 0, 0, -3, -4
stat_mental 13, disciplined, highly_trained
stat_charge_dist 30
stat_fire_delay 5000
stat_food 60, 300
stat_cost 1, 1809, 603, 181, 181, 1809
ownership romans_julii
and replace with: (the red text)
Code:
type roman triarii
dictionary rtr_roman_triarii
category infantry
class spearmen
voice_type Heavy_1
soldier roman_triarii, 35, 0, 2.0, 0.45
officer roman_early_centurion
officer roman_early_standard
officer roman_aquilifer
mount_effect horse +3, camel +3
attributes sea_faring, very_hardy, can_sap, hide_forest
formation 1.0, 2.0, 4.0, 4.0, 5, square, shield_wall
stat_health 3, 3
stat_pri 13, 1, no, 0, 0, melee, blade, piercing, spear, 0, 0.4
stat_pri_attr no
stat_sec 0, 0, no, 0, 0, no, no, no, none, 0, 1
stat_sec_attr no
stat_pri_armour 10, 11, 6, metal
stat_sec_armour 0, 1, flesh
stat_heat 4
stat_ground 0, 0, -3, -4
stat_mental 13, disciplined, highly_trained
stat_charge_dist 30
stat_fire_delay 5000
stat_food 60, 300
stat_cost 1, 1809, 603, 181, 181, 1809
ownership romans_julii
Other lines may be different. But it's okay - just make "no, no, 0, 0, ...." to clear out their secondary weapon and they'll stick with the spears.