> You can get a server with a terabyte of RAM for a price that's insignificant compared to the cost developing software to run on a cluster.
This assumes that a) You're in the valley where average developer salary is $10k a month or more, b) You're a large company paying developers that salary.
There are lots of other places where a) Developers are cheaper, or b) You're a cash strapped startup whose developers are the founder(s) working for free.
Comparison still holds, because if you buy a cluster with X amount of RAM the price will be roughly the same as a single server with X amount of RAM. Except that for some large X there won't be any off the shelf servers you can buy with that amount of RAM (let's say 2000GB), but lets be honest here, 99% of companies needs are under that X especially if we're talking about startups.
This assumes that a) You're in the valley where average developer salary is $10k a month or more, b) You're a large company paying developers that salary.
There are lots of other places where a) Developers are cheaper, or b) You're a cash strapped startup whose developers are the founder(s) working for free.