I write stuff~?
A bit of distance, if temporary, can bring people a lot closer. Thomas needs the opportunity to grow as a person. If he remains on the Hind, he'll almost always be in the shadow of his Heroic Spirits. After all, once they gain bounties and ascend, they'll become even stronger.
No. Even if you re-read, not even half the chapters mention how 'useless' he is. Saying 'most' is a gross overstatement to exemplify how you feel about the novel. Now that he's no longer in the shadow of his Servants, he'll be able to grow a lot more.
That's why Drake wants him to leave. So he can grow into his own without living in their shadow.
All of Thomas's Servants have Independent Action except for Caenis. However, as mentioned by Drake and Chiron, they can replenish Mana by killing and accumulating belief/faith. Caenis can also draw Magical Energy from the Equivalence Engine, so being separated from Thomas for a year isn't a big deal.
It's more like he will be accompanying them during their travels through the first half of the Grand Line. If Thomas remained with the Wild Hunt, there would be no real tension. Most of his time would be spent training and planning his future actions rather than actually doing anything. This gives him a chance to grow organically while also pitting him against foes around his level.