My question is kind of related to the original. Is there a reason to give out titles besides when requested? Is there a limit to the number of direct vasssals you can have, or a reason why you would want to keep that number small?
1. For kori, it's risky to hold more than 6 or 7 on your own - the revolt risk is simply too high. Getting down to 6 kori or less in your personal desmesne is wise in this game.
2. Rewarding a particularly important vassal by making him a Kokujin or a Daimyo can cement his loyalty to you and ensure that of his immediate heir. Through judicious granting of titles, you can do what the leading Samurai did historically, and build an heir or a close ally up, raising their opinion of you and keeping them loyal as a strong "right hand".
3. You can have as many vassals as you want. However, grooming a "right hand man" and making him a fellow Daimyo can let you keep your own court manageable, while building his up. Ambitious samurai in your own court will constantly petition you for kori and compete with one another for honor within the clan; they may or may not support your designated heir, desiring leadership themselves. Having a strong partner can alleviate this, and protect against rebellion.
4. More vassals means you are winning, so from that standpoint, they're better than small ones. However, certain characters, especially ambitious ones, will not remain content with no opportunity for advancement. Eventually, they (or their heir) will either leave your clan and go join another, or revolt and start a civil war, if overlooked for too long. It is important to monitor their personal honor; if it gets high enough, they will breakaway and declare their independence to become Daimyo if they cannot do so under your banner. Part of the challenge of Sengoku is finding the right balance.