Instant auto destruction should not be possible, as was intended from the very beginning.
Like that, it would give too much value to micromanagment, where you could have an anemy build surrounded and self-destruct units as soon as they reached, for example, 50 HP. It could also give unfair advantages to people using modified/cheated versions of a client that would give those units automatically the self-destruct order.
Like it is now, you may order to self-destruct a unit just to find out that it would've last longer if only you've not done it or to find out that you gave the order to late and the unit dies too before it can explode This requires strategical/tactical control of the situation.
Also, we can think of the timer as the time needed for the unit to overload it's reactor into the point where it explodes
And the expression
Zoombie wrote:... the GLORY BUTTON!! ...
is extremely funny and inspiring.