Slow computers suck time from people without them knowing it.
OS X makes very good use of RAM to largely eliminate most delays from hard-drives when it comes to opening and running apps, and it'll even preload stuff now.
This means that the main advantage for SSDs are on start-up times or accessing larger content like games at movies, however, for start-up times you don't really
need them to quick for a desktop (you don't
have to stand around waiting for it), or you can even just put it to sleep instead in which case the delay is eliminated entirely. Games aren't really a target demographic for the Mac Mini, and are more of a niche use-case regardless, so those users are free to use an SSD if they want. Movies meanwhile don't require SSD speeds unless you're trying to edit them, and even then if you're not doing serious HD editing a hard-drive is still plenty fast for the purpose.
Spinners had their day, it is now SSD's turn.
Sure, when those SSDs can provide the same capacity, at the same cost. Otherwise you're trying to convince someone to buy an entry-level computer with limited storage at a higher price, with the only advantage being speed they may not actually see any benefit from.
Spinning drives still have a clear advantage on capacity and cost, and in an entry level the latter is more important, particularly if the speed is plenty for the typical user's needs.
I am not the only one. A little company you may have heard of has like 75% of their computers SSD. Yup, Apple!
Apple has SSDs as standard in their laptops and the new Mac Pros; the latter has them because it is a performance machine, and since anyone with significant capacity needs outgrew internal storage long ago anyway, so it makes more sense to have a faster working drive, plus a high capacity external drive for storage.
In the case of laptops speed isn't the only advantage of SSDs; a lack of moving parts is a big bonus for mobile devices (no wear and tear), as is the lower energy consumption thus longer battery life. Even so, they've lost capacity as a result, but then they're not a device where someone would typically store a huge movie library anyway.
So I say yet again; SSDs still aren't cheap enough to make any kind of sense being forced into an entry-level computer, as making it more expensive would be a mistake. So why exactly do you seem to be so opposed to the idea of letting customers choose for themselves whether they want to pay more for SSD speeds or not?
