Why would you ever choose open-cell spray foam?
Like most things, both have their benefits depending on the situation. Personally I'd default to using closed-cell unless there was a specific reason for open-cell, which can include:


Open-cell permits drying. If you already have a vapour barrier (eg: below attic rafters) this is good, as multiple vapour barriers can trap moisture and lead to problems (eg: in the wood for the rafters). 
Open-cell remains flexible. This can be useful in some cases where the house is settling or shifting. Closed-cell does resist this to some extent, but when it fails it's likely to crack and leave a gaping hole, which compromises the insulation value.
Generally cheaper per same RSI. You'd have to price this out to be sure, but it uses about 1/3 the material for a bit over 1/2 the insulation (RSI) value. 
Higher expanding. As an example, I used closed-cell spray foam to do most of my basement, but open-cell to fill the joist bay for a 4' cantilevered floor, because it wasn't possible to reliably get closed-cell to cover the bottom of the floor above. Using open-cell allowed the entire bay to be filled completely.
Less off-gassing than closed-cell. Most closed-cell uses a hydrofluorocarbon (HFC) based blowing agent, while open-cell uses a water-based agent. This apparently leads to less smell, if that's a concern. (In my experience, having used it in two different houses for different applications, smell and off-gassing of closed-cell was a non-issue. If you search the internet for this though, you'll find lots of complaints.)