When I said "seeks are becoming more expensive" meant in relation to other things going on in the system, not in an absolute sense.
From a programmer's perspective, the growth in hard disk capacity has not been matched by a corresponding increase in either throughput or worst-case latency.
Because hard disk throughput has not kept pace, in a high performance setup, your only hope for reasonable throughput is to use RAID with striping. But RAID increases the minimum size that you can read-- before, that minimum was a sector-- with RAID, it's a stripe. This makes hard disks even less of a random-access medium, since you never want to be reading just a few bytes-- you want to read a whole RAID stripe at a time in order to be efficient.
Most programmers don't know about these details because the database does all this for you.