Earlier quoted context omitted.
The same is trivially true for a dozen nations. Should we attack them all ? Hell, one might even say it's true for America itself.
Maybe crude to answer my own question but this seems a pretty good answer : http://www.reddit.com/r/TrueAskReddit/comments/1lw8yg/why_do...
However, everything changes if Assad regime falls and rebels - among which would be such organizations as Syrian Islamic Front or Syrian Islamic Liberation Front, with predictable amount of love for Israel - were to come to power. They would have no experience of losing the war like Assad regime had, and no problem with suffering a bit more of mayhem is it's for the good purpose - after all, that's just what they did right before it and succeeded. And they'd own all munitions that Assad regime had assembled (at least those that left after the war, but there would probably be sizeable amount left, and if not, Putin could help), including WMDs. How there's any benefit for Israel in that?
Israel would be glad to have Assad's ass get some thorough kicking. However, having an explosive unstable government with WMDs on its borders is not exactly one's dream. Israel already has this problem with Lebanon, and it didn't work that well so far. Having second one is really not anybody's wish.