What's the issue with using Trello for clients?
At a glance, a client can see exactly what is being worked on, and where the status of each of the steps of the project are at. The client can see who is working on something, and provide them with feedback. You can @John the client and get their attention to particular card/list/upload.
Your post makes it pretty clear you use Basecamp mostly for to-do lists. Well, Trello is to-do lists on steroids. 37 Signals has barely updated Basecamp since it's inception.
It's more than just to-do lists though, I keep everything in here. Like I said earlier too I already have clients/VA's in here and it would be a major pain to move everyone.
For my business needs Trello just wasn't great, I gave it a try for a couple months and then I upgraded to Basecamp -- which I'm much happier with. Certainly not knocking it as a program. If you check my post history I was actually a big promoter of it

What's funny is I think some of the same people I recommended Trello awhile ago are telling me to switch there.
This thread was less about arguing about why I should switch off Basecamp though. It's about automating and how people set theirs up.