定语从句point之所以用where是因为先行词与从句之间需要一个介词
on the point前面的on与定语构成介宾关系
This is the point where we argued.
这里的where相当于on which的意思
同样的次还有situation,case,activity等
下面看一下09年上海高考吧：
Mozart's birthplace and the house -----he composed"The Magic Flute"are both museums now.
这个当年失分还是比较严重的,很多同学认为 Mozart's birthplace and the house是两个先行词,所以选用了that.
事实上he composed"The Magic Flute"are both museums now.
这部分定语是修饰the house,对于he composed"The Magic Flute"要加in
the house,所以答案选用on which（相当于where）