Fling is definitely an interesting option, but it is quite easily outclassed by moves that can inflict similar or better effects, and possibly with more reliability.
By using Fling, you are pretty much forfeiting your items slot, something Slaking values as it needs to deal as much damage as possible before it's next Truant turn. It also means that if you don't use Fling, you have a nasty item that will only harm you going forward (a Flame Orb would be completely counter practical with the set you're using). Simply using the move Toxic would mean you need to waste your items slot.
Also, how would Fling make a difference? The person can just continue using Protect, which will protect them from Fling and its effects. Moves like Fling are better used by Pokemon that can use them correctly; some examples are:
- Sigilyph and Psycho Shift + Magic Guard. Sigilyph would hold a Toxic or Flame Orb, and would use Psycho Shift to pass the status problem to its opponent. Magic Guard would prevent it from taking damage. With this set, Sigilyph also becomes a great status absorber.
- Choiced Pokemon, particularly Scarfed Pokemon. They can use Trick or Switcheroo on a wall that it using a move like Thunder Wave to lock them into it and essentially make them useless for the rest of the match. You will also get the opponent's item, which will often by a handy Leftovers if you use it on a wall.
Just my thoughts.