One of the more common problems with sump pump failure (besides the AC mains being off) is the pump itself has an on/off switch failure.
This makes it useful to have a 2nd pump as a redundant measure and that pump being DC, battery powered, kills two birds with one stone.
The weak link in that scenario, in my experience has been the SLA battery charger. The contemporary highly efficient (and consumer grade, cost effective) SMP PSU designs in those, save you money on your electric bill but have more possible faults. I suppose the bottom line is nothing lasts forever or is cheap if it's good.
Considering my flood damage losses if there were a fault, I didn't think for a second about an inverter for AC loss instead of a redundant, second DC powered pump. Plus, consider that if the 2nd redundant pump is mounted higher than the primary AC pump, if there isn't a failure it won't ever be submerged so won't ever run, so its lifespan will be much higher as a result, BUT as with all mechanical things, it is a good idea to manually trigger a backup pump to spin the motor a few revs, to verify that it works and not let various aging factors cause it to seize.