imo the idea of not buying points without a specific redemption is to avoid stockpiling points...many of the crowd discussing this are folks who already earn a good chunk of points naturally so there's a steady flow of them without purchasing on top of that. But if YOU know you're gonna use them relatively quickly and your points balance won't ever get too high, then I think the conventional warning does not apply to you.
Do keep in mind what wasknown said about adjusting your calculation to account for lost points earning from cash bookings. For elite members, and especially during good promos, this can be around 20%!