It's not as fast, no. But the advantage of being able to work year-round here (the Great Plains) makes up for it. Additionally, you can often thin it a bit to help make sure you don't lose any detail. I had a couple bad experiences with spray primer: once I was too close and sprayed too heavily, and the other was humidity related. Once bitten, twice shy.
The liners have an additional benefit, as well. With the color selection available (blue, grey, brown, red, green, and sepia), you can do another bit of ground work to help your base coat or work with colors that typically don't cover as well. Reds and yellows are often tricky and require multiple base coats to get decent coverage. By using a brown or red primer layer, you can help provide a "friendly" base to help out with those shades.
Heh. As much as I'm looking forward to April, I'm not really looking that far ahead. I have Reaper's Bones IV Kickstarter coming in February(ish), many leftovers from Reaper's Bones III, and a serious backlog of Imperial Assault miniatures, so I've got plenty to do in the meanwhile. That being said, I'd worry more about the miniature production than some of the other components. So while I think April is good, I wouldn't be shocked by May or June.