Since most people judge computer speed based on the time it takes to boot and the time it takes to load applications, you are right, the SSD should be used for those things. For users who boot up once a week or less and launch applications about as infrequently the decision is cloudy. I know that my reboots and program launching is infrequent. Where I really see the improvement is in programming -- my 2009 mini with an SSD is about 5x faster than a company-supplied Windows box with an Xeon processor, hard drives in RAID-1. But on the heavy processing extreme -- Handbrake or FCPX/Compressor, these are CPU limited so the drive doesn't matter again.
There are no simple, blanket answers to this question.