Do we have a "powergamer" among us here. I mean when did spearman/pikemen become useless. If a group of knights come, I rather have have a group of pikemen than macemen or longbowmen when I combat them, to use knights on knights is to much of a gamble for me. (And yes, a balanced army is the best).
Don't the AI use marines? I was attacked from the sea by 7 marines (and just as many other units). That was quite devastating. Can admit that the AI doesn't only use marines when he makes amphibious assaults, but he does use them. And before I could counter him, he has transported in more unit (there was over 30 in the city, before I could muster up enough units) and it was a blend of all modern units (incl. other mentioned "worthless" units).
If someone attacks a weak unit, then you should guard it. So if you have a carrier, guard it, the AI does (if he has any ship available, that is).
Regarding the explorer/medic: He is available before Red Cross and just as mentioned above, he is attacked last and he has a move of 2 (and can join some cavalry raiders, if you like). Yes, I use the Red Cross to (usually building marines there, and destroyers if I can).
PS (off topic)
The above mentioned city (the one with over 30 modern units), I used a rather odd tactics to take it back, I nuked it (yes I nuked "my own" city). Not many defenders was left and they ware injured. Did I get some bad 'Rep' for that, and if I didn't, shouldn't I get that.