That's it, but I don't want to pay for 2 batteries
Here's a 3 year old article about Li-poly vs Li-ion:
Very low profile — batteries that resemble the profile of a credit card are feasible.
Flexible form factor — manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
Light weight – gelled rather than liquid electrolytes enable simplified packaging, in some cases eliminating the metal shell.
Improved safety — more resistant to overcharge; less chance for electrolyte leakage.
Lower energy density and decreased cycle count compared to Li-ion — potential for improvements exist.
Expensive to manufacture — once mass-produced, the Li-ion polymer has the potential for lower cost. Reduced control circuit offsets higher manufacturing costs.
The Li-ion polymer offers little or no energy gain over conventional Li‑ion systems; neither do the slim profile Li-ion systems meet the cycle life of the rugged 18560 cell. The cost-to-energy ration increases as the cell size decreases in thickness. Cost increases in the multiple of three to four compared to the 18650 cell are common on exotic slim battery designs.
If space permitted, the 18650 cell offers the most economical choice, both in terms of energy per weight and longevity. Applications for this cell are mobile computing and video cameras. Slimming down means thinner batteries. This, in turn, will make the cost of the portable power more expensive.
I guess it seems more feasible, knowing this, to me now that the ultrabay battery can last just as long as the normal one.