If someone...

[420]

If someone compromises the BTC code; like a loophole to doublespend

Would all other altcoins be vulnerable to the same defect?

[DannyHamilton]

It would depend on the compromise/defect.

[RyNinDaCleM]

It would also depend on if the alt-dev noticed the hole, which code was forked, and what code was used/removed/modified. Somecoin that uses the (bad) forked code, and changes nothing but the name, then yes, it would be present.

[420]

So far I've never noticed anyone hear of the hole.

At least the benefit to more alts is if someone finds a hole in bitcoin or another hopefully someone notices the 'cheating' and the other coins can react to avoid it but maybe there won't be enough time if the attacker can just switch coins; although bitcoin would be the most profitful to attack; the others might be good to attack so that people don't jump to them. If he wants to ruin alternative currencies