The very same people telling the OP to trust your fabricator would be blaming the OP for not properly vetting the fabricator if he does a bad job. Whether or not you like it, there are bad actors out there.
Since we had a post here about mechanics, let's remember that once every couple of years television news programs will pull out auto mechanic undercover investigation and they almost always find that half or more of the repair shops scam consumers with unnecessary repairs.
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It is entirely possible that there is a reason that the fabricator can't do it the way you propose. It is also possible that the fabricator is simply on a different page with the project than you are and some communication will help get everyone together. However, there is a solid chance that the fabricator is taking advantage of you.
The fabricator should be willing to communicate his/her reasoning with you in an understandable way. If the fabricator is not willing to do that, then you should find someone who is. In my experience, honest contractors have no problem informing the consumer and the people who refuse to explain are almost always the people who have no explanation.
Thus my advice is ask the fabricator to walk you through the number of slabs he is recommending rather than you suggesting cuts. If he refuses to do so, then statistically, you are better off finding someone who will.
Q