Ah, I didn't know they'd already released Spearhead From Space. If I'd really wanted Terror of the Autons enough to buy it and not Spearhead From Space, then I'd either see if someone's selling the one I want on ebay, or buy them both and sell the one I don't want on ebay.

I think the DVD releases are assuming that people are buying them anyway. Which isn't the case for people like me who rent them. If I really want to see the extras and am interested enough to listen to the commentary.