Need and being able to afford are two different things. Even 'need' has to be taken into account if you 'need it', that is no other way to do it.
If I'm going to use a computer for 10 minutes, a 3 minute boot percentage wise lengthens the time I'll use the computer considerably. However, if I'm going to be using it for 7 hours, insignificant time addition with respect to the work I'd do.
As for SSD reliability, well, that to me is another aspect to think about. WRITING to the unit shortens life, period. All SSD's have 'extra' cells and allocate them as needed. All gone, drive is too. Yeah, as I'm using ours now, careful where programs and OS can write too, I'm sort of extending the time to well after we're done with the computers, but if I had ONLY SSD's, I can't say that would be true?
Not everyone can afford that larger sizes needed to fully replace the mechanical(s) they now have. To me the small gain in time of a program loading and being able to be used in relation to the time I'm going to use the program is insignificant and meaningless.
SSD's will become more standard in the future, probably the combo of a small SSD acting as cache with a large mechanical behind it will come first (and some vendors supply that now).
What would you rather do? Spend $$$'s on SSD's or Intel Extreme Edition CPU's and large RAM Graphics cards????