On ours the Battery has a 5th motor to drive the footrest,if you spec an ECO with a 5th motor the price is very close so the only downside to specifying a Battery is they only last about 3 years and need replacing.
The upsides are that the chair can be used if the cleaners rip the 13A plug off for a while whilst you can replace it, you can test the chair without mains present which is great for fault finding in the workshop and the chair can be used in a corridor if necessary e.g. acute admissions units and no beds free..
Hope this helps,I agree they are very rarely needed for mains failure.
www.healthtec.co.uk