Until it has an API I don't see many people wanting one.
While the appearance of the API will undoubtedly be very welcome, if I may say so, you have to step outside the mindset of the hardcore hobbyist whose ranks are (unsurprisingly) over-represented in this forum.
There is a substantial proportion of Davis buyers who appreciate all the extra features that computer processing brings (eg see your own current weather on a smartphone app from anywhere) but who really can't be bothered with all this running a PC, setting up a website etc stuff. Products like WLL are a godsend to them and they will buy one - they really don't care too much if wl.com is the only easy place they can view their data and many will feel that the $50pa Pro plan is a necessary evil that they're willing to entertain, just like they do for many other subscriptions they pay for these days.
I think there may also be a significant number of hobbyists who might run a WLL alongside their current logger-based solution just for ease and flexibility of uploading to wl.com.
So personally I think it will sell, certainly as much as the IP logger and probably more so.