My question is, is it worth buying the regular version of AQ when you have ordered AQ:I kickstarter?

Worth is kind of in the eye of the beholder (it's subjective). They're each their own game/campaign and may interact well with each other, but if you don't like AQ:I or are not craving more, there wouldn't be a reason to purchase more.

I'll throw a dissenting opinion: not worth it.

AQ and AQ:I have similar game mechanics. AQ:I is slightly more complex with its Damnation mechanic.

If you have KS AQ:I, you will have lots of heroes, so the heroes from vanilla AQ are just drops in the bucket.

I would go further and suggest something different:

Wait.

You're getting AQ:I, the KS edition. Play that several times. You don't need any original AQ stuff with it (except for perhaps that fall of arcadia campaign). But really, you'll have a TON of stuff to play. After you've played a few campaigns, you'll know if you want more scenarios or not, because that is what original AQ will boil down to - More scenarios. (well...more heroes too, but you already have a metric ton with a KS AQ:I).

Many good suggestions so far. Waiting until you have a better idea is wise. You will probably get to the point where you don't need to ask: you'll know whether you want it or not.

But also, if your friend has it, then just use their copy. In my gaming group we share games quite happily.

AQ for you means another campaign + upgrade deck. The extra heroes are a small bonus compared to the tons of heroes you got with this KS.
Since you played it already, you will be able to tell whether you really prefer a different setting (Orcs, Goblins, Minotaurs) and a slightly simpler campaign.
Personally, I think AQ is great to introduce new players to the game, while AQ:I is the next step (but hey, I haven't played AQ:I yet).

I also like Beyond the Grave the most and I'm happy I bought it as well.
Keep in mind that Beyond the Grave is meant to be played with AQ and not with AQ:I. So you'll have to buy AQ in order to play BtG.