I have close to 200 pages of conversations that need to be whacked. No problem sez I. I'll just whip into AdminCP and delete 'em all from my profile. Imagine my surprise that the option isn't there for Admins.
Please add this option. Managing conversations is a pain and without this option, just became a nightmare.
What exactly are you trying to do? The ACP option is not the same as leaving a conversation; it actually deletes it, as if it was never sent. It removes it from the inbox for anyone that received it. It's generally designed around spam and abusive use.
Users leave conversations, meaning they no longer appear in their conversation list. The conversation still exists for other users. Once all users leave the conversation, it is deleted. It's a good system in that there's just one conversation for all users rather than multiple copies of PMs as there are in other forum software.
Whilst this doesn't solve your current problem with 200 pages of conversations, for most users there is already a suitable way of managing their conversations regarding leaving them. Selecting the conversations in the inbox and then using inline moderation to leave them en masse:
For the time being, you could temporarily increase the Discussions per Page value to a higher value so you can select more conversations in one go without changing pages before mass leaving them, though it's probably best not to leave too many in one go in case it causes performance issues.