Re: Aikido against very high kicking (like in tae kwon do)
In our dojo, we practice extensively against mae giri or kicking attacks, high kicks, low kicks and all other kicks in between.
In my experience, high kicks are generally less effective than say a low thigh kick. Again, we emphasize ma-ai and irimi when dealing with kicks. I think it is good practice for aikidoka to learn how to defend against kicks because other martial artists who are very skilled in kicks can cause serious injury from the force of their kicks and their sheer speed when delivered by a skilled MA makes them very difficult to anticipate.
We had a visiting yudansha once and he was kind of shocked when during randori he was attacked by multiple kicks by one of our resident TKD expert. Needless to say, the speed and execution of the kicks left him with little options except to try to parry the kicks which is inherently dangerous because of the power and weight behind the kicks.
IMHO, there is a serious dearth of practice/techniques against kicks in aikido and I feel that this is a serious gap in aikido repertoire. Again, this is is just my opinion.