Even if Rand didn't channel the True Power, they still had him in hand.

However, there are a number of reasons why I think the Shadow would prefer this route to capture. For one thing, it's probably unsafe to capture Rand. Think of the disasters that have followed for the Shadow whenever Rand was captured. Getting him in the Shadow's hands might just terrify the world into uniting to save him, for example. You also can't risk killing him. I'm not certain why that is, but it is evident the Shadow doesn't want Rand dead, and given that prophecies predict the victory of the Light with Rand's death, I suspect there might be reason for this.

Of course, there is no certainty in any of this, but I think Shai'tan might have thought that making Rand dependent on him directly with the True Power addiction might have been the best solution.