They will not slip out as such but when the car is jacked up they can become loose as they are shorter, this can fail a mot.
Your better off fitting a set of shocks as well really, if you look at the work involved it is not worth doing it twice.
I agree with wayne, after me and my brother fitted eibach springs today on my golf, i really cba having to do it again, it was quite a bit of work but i will have to when i put new shocks on ha! at the end of the day up to you lad!