Yes, the sales side of the business is critical. So is the engineering side, and the product side, and the support side. In a startup everything is critical. Which is why an equal share is the only reasonable approach; otherwise you're arguing over which of your organs is more important, your heart or your lungs or your liver.

If your founding team thinks "sales is less important than engineering" and thus deserves a smaller share (or vice-versa), you're already set up to fail. I went through this once with a CEO with an outsized belief in his contribution and I consider the lesson learned.

I'm sure there are plenty of exceptions to the "equal share" rule, like very junior engineers - but you might want to consider whether that's a good idea for a startup.

I look at this from the other side, btw. In my current startup I had the moral authority to demand an asymmetric split, but I chose an equal split anyway. We're going to be at this a long time. Day 0 is a dumb time to get greedy.

Or look at it this way: The other founders in your company got greedy and missed out on an opportunity to bring you on board, and your contribution may have been the difference between success and failure. That's their fault, not yours. I hope they've learned their lesson.