So many tasks can only take advantage of a single machine (and too often, only a single thread!). While that doesn't change the cost of the CPU time it uses, it

So many tasks can only take advantage of a single machine (and too often, only a single thread!). While that doesn't change the cost of the CPU time it uses, it does mean that you have to wait much longer to get those N hours of CPU time to save that hour of programmer time. That wait time could completely undo the productivity gains.