I think UAD could do similar and offer things like ‘3 months free’ with the purchase of a quad, or 5 months with an Octo. The problem for them is how much that then positions their ‘brand’ alongside others that they would currently consider inferior offerings. They do offer the ultimate bundle so there thinking isn’t miles away from the rental space in terms of having an ‘everything’ package.
In terms of what I do, I have Slate on rental and everything else bought. For me it is about how many months before I would have paid for all of the parts I would have otherwIse bought. So, looking at the total cost of ownership. Like @sight says on rental you have to keep paying as you only own it for the month, until next month and the next. With Slate, I could see they were continually adding things. That has to be a big part of the rental argument.