I respect your opinions on this, and if I was playing with a DM that ruled this way, I'd have no problem with the ruling. What I meant is that the spirit of spring attack is to allow the character to spring in, make his attack, and spring out. The spirit of haste is that it gets you moving so quickly you have time to do an extra partial action in the same time it takes to only do your standard 2 actions (move-move, or move-standard). So therefore, both the DM and I agreed that moving in, taking the standard swing and the hasted swing, then moving out fell into the spirit of the rules.
Part of my reasoning for this is that you are splitting up your move-equivalent actions into two smaller moves. Logically, there is no reason that you couldn't take your second half-move after the hasted action. Rules-wise, maybe not. We decided on it during a battle that the DM had augmented heavily, and a couple of extra sneak attacks early on made the difference between victory and possible total party annhilation.
We also both agreed that, yes, springing in, getting the full attack action, then springing out, would be insane and too much.
But this decision isn't unbalancing, particularly when we are going up against Marilith demons wih 6 arms, extra levels of fighter, an AC of about 50 or so, the perfect multiattack feat, hasted, and each arm wielding unholy, bane weapons. We all have some potent magic items, but the DM equips the bad guys every bit as heavily as our party.
If you don't want to allow this in your games, fine. If I ever come to your house to play, I'd abide by your ruling, no problem. However, my original question in this thread is how you would handle the lingering damage feat on a character with multiple sneak attacks per round. So far, the DM and I are discussing allowing only the first successfull sneak attack of the round to linger into the next round. What do you think?