Am I the only one who likes this feature?
One of few people, I imagine

I like it sometimes, when I'm strong and am able to tap into a large alliance network. I did it as Rome once, managed to get into a Greek alliance that included the Seleucids. Needless to say, the Sels never received a single province in a war after that and I ended up gaining half a dozen vassals from her various wars.
Didn't Machiavelli write something that a state should always chose a side in a war because staying neutral was worse and other nations wouldn't then trust the state? Or something like that.
I'll admit that I haven't read much Machiavelli, but wasn't
The Prince written as a vision of Mach's idealised state? In which case, this idea of nations needing to choose a side is more about what he believes
should happen, as opposed to what
did happen.
What happened in history was a country that came under attack would ask any friendly country to come to its aid. What didn't happen was for all of the second country's mates to join in, and their mates, and their mates, and so on. When Rome and Carthage went to war, Carthage more than likely called in its (tributary) allies in Spain, but that wouldn't have resulted in all of Spain at war with Rome. When Rome and the Aetolians went to war, the Aetolians called in the Seleucids to help, but the latter didn't call in the rest of the Greek world to help them out.
Plus, as Nachinus pointed out, the cascading alliances issue can result in some absolute absurdities, like a country being involved in both sides at the same time and so on. Not hugely common from what I've seen, but it can still happen.
And monarchy's are inter-marrying so why wouldn't they come to their cousins aid.
Didn't always help, just look at the Seleucids and Ptolemies. Their family tree (which really resembles more of a mosaic circle than anything) didn't stop them from being almost constantly at war during this period.