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Ideally a grid charger is used from day one in a brand new car. Honda's battery management doesn't do as good a job as Toyota's at keeping the cells in balance and as they are used, the cells will begin to drift apart. Drift enough and some cells will be nearly empty while others are nearly full. The cells at either end are abused by over-charging and over-draining, causing early failure.
A grid charger gets the cells back in balance by very gently overcharging all of the cells until they're all full. This isn't great for them, but it's a far lesser evil than letting them get out of balance.
I was charging my healthy pack 2-3x per year, preventatively. On a battery weak enough to throw an IMA light you might have to charge as often as once per month to keep the light off - these packs have cells damaged enough that they very rapidly get out of balance. Once the IMA light is coming on more often than this, replacement is imminent.
In a mild climate, a regularly balanced oem battery might last many decades. An already abused and failing pack might be limped along for years.
The lithium pack is simply intended to be an alternative to the very poor quality NiMH cells on the market, which will function in the same way. If you can plug it in at night and turn the car into a 150mpg+ PHEV that's just icing on the cake. I believe this is still TBD. I've heard of people who grid charge their NiMH batteries every night and get boatloads of assist, resulting in fuel economy too high for the car's computer to track, but this is detrimental to NiMH battery lifespan. It wouldn't be for a properly designed lithium alternative.
Edit: you can build an effective homebrew grid charger for as little as $30. I have plans in my Insight thread, linked in my signature.
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