Pineapples are a better option for removing the last few bloons that squeeze through than Road Spikes because the price is 5 money cheaper. However, it should be timed correctly and should not be used against Black and Zebra bloons as they are immune to bombs. In BTD5, there is a premium upgrade called Grilled Pineapples which allows the pineapples to take only one second to explode.
Along with Road Spikes, Pineapples can be a lifesaver if the bloons start making it through your towers.
In BTD3, BTD4, and in BTD4 Expansion, you can slow down lead bloons with Monkey Glue, so it can be easier to pop them with pineapples, decreasing your chance of missing.
Pineapples are good for exposing and/or thinning out camo bloons or lead bloons. However, the exposing out of Camo Bloons only works in BTD4, where Camo Bloons were a separate type of bloon, rather than a type of bloon status.
In BTD4, BTD4 Expansion, BTD5, and BMC, the first upgrade for the Monkey Ace makes it drop pineapples at where the plane is at certain intervals. (First upgrade for second path in BTD5 and BMC)
Pineapples in BTD3 may pop 2-4 layers of the bloon.
Starting in BTD4, they can only pop 1 layer of bloon, seemingly being nerfed.
In Bloons Monkey City, When you click on a Pineapple Stand it will be missing the pineapple word at the back. (May be a Bug)
In Bloons Monkey City, when you get +1 pierce due to activated ability knowledge, the pineapples will explode instantly when they make contact with a bloon.
This is one of three fruits in BTD4 and 5, the other being the banana and the grape. Coincidentally, both pineapples and bananas are worth $20 (without the Valuable Bananas upgrade). However, Pineapples cost $20 and bananas give $20.
In BTD5, they are called Exploding Pineapples.
Unfortunately in Bloons Monkey City Mobile and BTD5 Mobile, the pineapples have a small blast and frequently misses a bloon eg. a lead bloon.