Hondas can be driven with a dead battery, while Toyotas cannot (generally), but you're going to find far fewer Toyotas with dead batteries because the range of charge/discharge is much smaller and therefore the batteries tend to last a lot longer.
I've tried driving my G1 Insight around with the IMA system disabled and it's a bit of a slug around town, but 50MPG is very conservative if you get a 5MT. 60-70MPG is more likely.
Some things to know:
The G1 Insight has a 10 year warranty on the battery. 2006 models can have it replaced for free until 2016, so you're not likely to find any screaming deals on them.
I just built a grid charger for about $30 in parts, which can greatly improve the longevity of the battery, and can be used to recondition a "dead" battery. The hybrid battery is only 1 year old, and I can reasonably expect to get 10-15 years out of it if I take care of it.
Replacement batteries aren't all that expensive, if you get a refurbished one. As tortillasoup said, most times only a few cells are bad and even those can often be revived with special equipment.