ERASE button.

This is a strange question...? There are lots of elements that are useless in certain situations. LOLZ and LOVE are useful in all situations. But it really all depends on your goal.

I would go as far as to say that there are no useless elements.

You could make an argument to say that the LIFE stuff is very duplicated because you only need two types and the rest are just for show (One for heat oscillators, one for static coolers) But aside from that, there are really no elements which do not get used. tip of the hat to the devs. However, if we are talking personal preference, there is ONE element which I would say is.... Not useless but is so inconsistent and that is electrode. You can only use two spots of it, otherwise it goes out of control. Thats the only element that I consider to be "useless"