You are right about the infrequent use. Better for the battery to be "topped-off" daily by the charging system than to sit and slowly drain during the week only to be "recharged" come the weekend. I guess this is where a trickle charger will do the trick and potentially extend the life of the battery.
Nevertheless, I was flipping through my service records and realized that I was sorely mistaken as to the age of the dead battery. I purchased it in October '97
so I guess 5 1/2 years is more than respectable, especially given the conditions of use. Boy how time flies. Bodes well for those OEM batteries.