yeah - got it. Even if you can afford the extra batteries, the pay-off diminishes when you are only cutting 19" wide so, you're right.Absolutely.
For just starting out, 6-8 extra batteries could last the mower for all my lawns for the day, plus the fact that they recharge quickly, PLUS the fact that the batteries will work with the mower, trimmer, blower and a hedge trimmer.
But the fact is, for larger properties, time is an issue. Someone can do an acre or 2 with a large zero turn and charge $60-$80 or whatever, while I'd have to charge like $300 for 2 acres because it would take me 6 hours on a push mower.
Of course it's not as time efficient as gasoline either, but I'll take the trade off any day.
Time will only improve the efficiency of these batteries, and I believe this is the future of lawn care.