[thejeff]

That is not true. (I suppose, depending on what you mean by "gameplay value" it could be...)

Plenty of people, including me, play SP and enjoy it.
Impossible AIs, especially with the BetterIndependents mod, can be quite challenging through midgame, if you aren't abusing the AI flaws or playing an early game optimized strategy (awake SC pretender, strong double bless nation, strong recruitable thug/SCs)

It's not as challenging as MP can be, especially as the game goes on and you get more options, which the AI doesn't even try to counter. OTOH, some strategies actually work better in MP than against the AI.