Truly, it depends on the story, and how strong the characters turn out to be. Maybe if the ending was different depending on the characters unlocked throughout the story, they would have larger significance. Though, if you're just putting them in to have them, it may not work out too well. In games like Fire Emblem, there were MANY MANY extra characters but they worked because you got them post game, and they just made for a fun extra boss run. Though, characters in Record of Agarest War made your team way too overpowered, and after the main story ended, you had like 30+ characters in your party most of which were very underleveled and you never ended up using them unless you wanted to spend countless amounts of hours fighting. Truly, in a turn based RPG, the best thing would be to only offer the hidden characters when the party's level is around this person's, and this person contributed to the story, and like add to the ending, or make something extra of the characters.