pentejr: Hmmm, but why waste juicy doubles that you could use to make home points with (having brought that 5 down to the 8-=point).
Have a look at the opening move rollouts and see how much splitting with the 1 is in favour. 5-1 as 24/18 is considered to be a blunder. (See Backgammon Links above)
The other aspect of splitting is that it gives two options on attacking the outer table or making a break for it.