A skilled player does not let the opponent pick up a big discard pile. In a game with skilled players it is not uncommon for the stock pile to finish and the discard pile not being picked up even a single time.
Pretend that you throw something garbage, the opponent does the same and discards a card with the same value but you kept 2 copies in your hand and now you pick up the discard pile.
If the discard pile is big and you cannot discard something good, it might be a good idea to first first meld everything you can so that you secure those points if the opponent pick up the discard pile and goes out.
Bluffing and reading the opponent is a big deal in this game. Did the opponent hesitate before drawing? Probably he/she has 1 card of the one you discarded. Does the player think a lot of time in their turn and has not opened yet? Probably the player is planning to go out in a single turn when opening and you should meld all you can - but it could be a bluff.
Knowing what cards are in the discard pile and trying to figure out what cards are in the hand of the opponent is very important. The faster you play the easier it is to remember.
If the opponent has many cards in hand then this is of course an excellent move.
It all depends on which cards you have in hand, the current scoring, the opponents hand and the state of the discard pile.
Yes, that is a viable option if you are going for winning the match with more than 100 points. But of course the opponent might go out at any moment meaning all the cards on your hand will be negative so it would be wise to at least meld most of it and keep some in order to keep picking up the discard pile.