One big point you forgot to consider is team comp.
If you're a melee comp that wants to fight but can't siege well, then playing around baron and dragon and contesting enemy attempts at these objectives can often be the best way of winning. If you try to push waves as a melee fight comp without someone who can tank the tower indefinitely, then you'll probably lack the form of hard engage and kill pressure needed to win a fight under tower. This is because the enemy will have time to poke you while clearing waves safely from under their tower. Your team would have to take tower damage, plus kill the enemy team from full health, all while your tank is limited in movement if he has to hold tower aggro should the fight split. It also limits who can engage first, as this is the easiest way to ensure your tank gets tower aggro. This can be extremely hard to win a tower siege in this case, and makes fighting for other objectives much more enticing as you don't limit your tanks movement, or take extra damage. In this instance, dragon and baron numbers become inflated, while towers can be daunting to take against even one or two defenders.
If you're a poke comp without much of a tank then taking baron can be very risky, and often just grouping and poking as 4 or 5 is enough to get everything up to an inhibitor tower. After this point, split pushing or hopefully the gold difference can allow your team into the base. If you try to take baron as a poke comp and either lose the fight, or the buff to the enemy team, you might throw the game altogether. Losing baron as a poke comp (especially an AP poke comp) can make pushing waves impossible, and can also provide more cover for the enemy team to hold a siege. In this case the first, third, and fifth dragon are inflated, as well as all towers, while baron takes a massive hit.
If you're a split push comp then it really comes down to what split push you're doing (1-4, 1-3-1, 1-2-2, 1-2-baron sneak, etc.) and what you think you can get away with depending on all other factors, as well as what would benefit your team's win conditions should they change over the course of the game. This can lead the numbers to swing rapidly on a game to game basis, if not minute to minute basis within each game. Maybe you know you're not going to be fighting much, but rather applying map pressure as 3, and then letting the other two split push down towers - guess what: Dragon 2 and 4 don't suck so much now. Maybe you want to group a bit more after the outer towers are down, but your Fiora is doing great at 1v1 or 1v2ing top - well then: ward up the enemy jungle, grab baron when it's safe, and go end the game, because baron is now the only thing you'll need to do so.
