It's the difference between being the sexual slave of a brutish orc who is just as likely to toss her aside once he's lost interest as he is to kill her at a random impulse. SoC has largely shown orcs to be cruel, savage, and indifferent to the pain and suffering they cause, Ulcro is no different. Plus it puts her in a scenario where she is most likely to survive long term and remain safe and healthy. At the castle she won't be in danger of being in an orc village in the middle of nowhere, which are plentiful. What happens when another band of orcs attacks, or a new challenger rises up to try and wrest leadership of the clan away from Ulcro? Ultimately the safest place is at the castle simply because until Rowan can either secretly scrape together a resistance of his own or find a way to topple the twins from the shadows, the least likely placed to be attacked is the castle.
It is
because Ulcro is a foolish mostly-brute that Delane is better off with him; she is a clever woman who easily uses his infatuation with her to accomplish her own aims (as she has spent her life training for, albeit with the expectation of political marriage to a
human rather than an orc), and has been granted a high status within the tribe that will allow her value to live longer than Ulcro. I wouldn't speak favorably of her position if she were just a toy like she is with Batri.
And this also assumes that she cares more about her safety than about her pride, which isn't the impression that I got from her conversations - she recovers a bit once she's in Rastadel and is able to play politics again, but when she was in the camp, she sounded like she cared more about the death of her family's influence than the danger to herself. She explicitly talked about what a thrill she found the "dance of hidden daggers" and "whisper wars of powers" that comprised politics. Taking her away from that by hiding her away in the castle would be killing what makers her
her, even if it saves her life.
It's true that there's a gamble to it - Ulcro wasn't guaranteed to overcome Batri, let alone future challengers. And Rowan's not guaranteed to win his war at all, much less without sacrificing enough of his allies to finish off that particular tribe of orcs. But again - she's a smart woman. If she sees that Ulcro's position is becoming genuinely endangered, she can start courting his successor - she won't have as much influence over him as she does Ulcro, but she'd already caught the attention of the lieutenants before she was hidden by Ulcro, and being very publicly by his side will only add to her appeal. Add in giving some good advice to solidify their position, and she's likely to retain a significant portion of her influence.
And if that's not the case, if it looks like the tribe's about to die out or his successor will be some hard-nose who doesn't care about what "advice" she can offer... Well, she's not in a cage anymore, you know? She can have a few orcs deliver money and luxuries to a location outside the camp, make her escape once Ulcro's on his last legs, and prepare for her next stage in life. Is it perfectly safe? No; the end could come faster than she expects, her escape could fail despite being in a trusted position now, or she might be discovered and robbed before she can arrange new protection. But whatever happens will still be on
her terms, rather than having to rely on the good will of others.
And like I said - Delane is probably dead as soon as she comes back to the castle, the same way the Twins went out of their way to capture her if Rowan sides with Werden. She's not just some random noble - she's tangible proof of Rowan's betrayal, and they'd be free to punish her as harshly as Andras desires.
