Does letting the battery go dead ruin the battery at all?

Yes... unless it is a high quality gel battery (and then, yes).

The lead from the plates drops off. Without seed lead on the plates, recharging the battery does not reform the lead on the plates. Thus, every time one of the many plates in the battery is barren of lead, the battery will not fully charge that plate. If it happens frequently enough, all the plates will not accrue lead and thus will not hold a charge.