When would you need a low bandwidth opamp?
That 27kHz is nothing. The LPV511 has a little brother, the LPV521, which has a gain-bandwidth product of 6.2kHz.  

They're not making it low-bandwidth on purpose. There is no real advantage to the low bandwidth, though it improves stability.
The low gain-bandwidth product is a consequence of the low power design. The LPV521 consumes only 350nA. You already mentioned slew rate, and it's closely related to bandwidth. The LPV521 has a slew rate of 2.4V/ms. To change the output level of an opamp fast you have to pump current to the output drivers. That's not what this opamp is designed for. Lots of applications are very low frequency, DC to a few tens of Hz at most. A typical application shown in the datasheet is a current monitor for a battery operated device, which will probably be near DC.  

Anyway, you'll have to pay dearly for such a bad opamp ;-). Seriously, even in large quantities the LPV521 costs more than a dollar, while you can get common opamps for 6 or 7 cents. It's that 500nW you're paying for. Try to find other opamps which will operate on a coin cell for 5 years or more.