The battery life for flagships DSLRs are pretty impressive indeed
$350 for the MB-D10 + MH21 charger is a steal if you ask me. Just that the battery life of the En-EL4 passed to you might not be up to your expectations already. Afterall, it is pretty old.
The battery life is something you should not belittle. When I went on trips that had no power source for me, I was glad I brought two En-EL4s along as it safely lasted me in the cold weather. Most of the time one of it ends up being standby and the charger is unused during the trip.
Another beauty lies in not having to worry about battery life while shooting. Or constantly recharging them before you take them out. On average, 1 En-EL4 can last me 1 full day of shoot with some life to spare. On another comparison, it can last me a week or so as mentioned on casual shooting, 300-400 per time maybe 3-4 times?
Package wise & price wise, $350 looks more interesting than $260. The En-EL4e is at about $180+ new. Even if one of it is beyond the life span, the other would still last you a long long time. It doesn't jump from 1 to 4 in that short time frame.