Look at Table O4-03 on page 55.
When you learn a style for an entire Weapon Group, you have to select one (1) of those weapons as the primary weapon (see also "Group Weapon Focus" and "Single Weapon Focus" on page 54).
The purpose of that option allows you to select a second weapon from the same Weapon Group that is associated with the style and use the full OB for that weapon. Ideally, this would be for a style that is learned for the whole group, and not for styles that focus on a single weapon (that being the original intention), but I don't really see a problem with allowing additional weapons from the SAME group being purchased in this manner -- it shouldn't upset things greatly.
For Example - Say you "Spear & Shield" Style is actually a "Pole Arm Group & Shield" style and you selected the Spear as your primary weapon. This option would allow you to add the javelin, pilum, trident, or a regular Pole Arm to the style so that you no longer get the -10 modifier to it or have to spend a week of game time making it the primary weapon.
Another example -- You have the Dancing Scimitar style, it is focused on the Scimitar (a weapon in the Long Single Edged Blades group). You want to add Falchion to the list of weapons that allows you to use foll OB for. You would not, however, be able to add a Long Sword to the Style. --- Yes the benefit is slightly greater here that it would be for the first example, but I don't think it would cause balance issues.
Does that answer the question?