Im already overspending so I might as well spend a little more to get better performance. If I had the option to upgrade memory or drive space later on I would cheap out upfront. The fact that I can't do that forces me to decide on maxing it out or wishing I had in the future.
That's different. You are looking at what you would gain for the money vs. just spending it because it is only a small fraction of the total spend. If you need the extra capacity then it is worthwhile; but some will just spend the extra money simply because it is small compared to the total spend.