You should add support for Real Time PAYE to Kashflow.
The only reason I'm looking at online accounting packages right now is because of Real Time PAYE. So is every other business in the UK that doesn't already have a suitable PAYE solution.
As such it makes sense to start doing my accounts online. What doesn't make sense is to have to use two systems when I can use one. I've found one competitor that can do this, Doesn't look as slick as Kashflow, but it does have it all in one place. Email me if you want to know who (I'm not going to use your support forum to advertise a competitor to you).
I've looked at the PAYE addons, but they aren't addons. They require you to use a different company to calculate your PAYE then update Kashflow, or they require you to actively use a different website and the export the data into Kashflow. Summary - they are not integral solutions. it's not all in one.
It's a huge opportunity you are not taking advantage of.