It's always best to come up with fluff interpretations that work with the rules. In my experience, it prevents frustration. And in D&D, you will surprisingly often find a good fluff explanation for the way the rules work. In this case: standard actions reflect your stance, your focus, not a set amount of time.
Having more than one attack each round doesn't mean you need less time per attack, or that you simply have more time, or anything of the kind. Time is not the issue, turns are. That means, what is at stake is how you use the opportunities that occur in combat.
In a melee, a first level fighter will find ROUGHLY one opportunity per round to get a hit in, and the rest of the round his swings go wild, are part of blocking, or are blocked in turn. A 6th level fighter is so good that roughly two swings per round are offensive-minded enough that they may connect.
Using two weapons, our 1st level Fighter can use that first opportunity to connect with both, most likely in very rapid succession, before the gap in the opponent's defenses closes again. It's still a lot harder to hit if you're springing to the attack with two daggers at the same time, but you may just score two wounding blows. The 6th level Fighter, using two weapons, can jump in there with both at the first opportunity just as well, but he may also find another opening a second later. This second opening can usually only be exploited with one weapon, though, unless he's extra special trained in the use of two weapons (Improved TWF). That's because it takes a special kind of balance to strike with two weapons at once.
So if you dropped your foe with the first of two weapons you wield, it only buys you a split second. You've already committed to attacking, so there's no way you can ALSO scan the battlefield for likely threats and adopt a posture that will let you react to one as it occurs.
At least, that's my take on it from a fluff perspective. The rules are simple and straight about the issue: ready is a standard action, as is attacking. The number of weapons you wield don't alter this.
Also, concerning Clerics rushing in to heal Hydra-bitten Fighters: The decision is the player's. The player's decision occurs within the framework of the rules, which allow you to know that the Hydra can't ready to attack the Cleric anymore. HOWEVER, you can also find good in-game explanations for that: the Hydra's focus was on taking down the Fighter. The Cleric was too far away to matter. Even if not all the Hydra's heads were needed to take the Fighter down, that fact is only just occurring to the Hydra as the Cleric rushes in. The Hydra's focus is sure to change once the situation changes, though (i.e. in game terms, when it's the Hydra's turn again).
