Strategy
In the Flash version, the bonus Bloontonium allows for a slightly stronger offense against an enemy city, and if your tanks are full then it can potentially be stronger than sending a normal MvM attack, without spending a single Bloonstone.
In the multiplatform version, the 20% discount to Bloontonium only matters for sending MvM attacks more often. Unless specifically wanting to maximize potential back-to-back enemy cities to keep sending back more MvM attacks so that the Epic Defender (2500 MvM defense wins) achievement is earned, it is generally not worth sending an MvM counterattack. The amount of Bloonstones earned from playing MvM games is often better spent grinding on game events such as Monkey Teams, which often earn much more valuable rewards for the same amount of time.
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