This is an example of where VC's interests and the companies are not aligned and VCs are pursuing short term goals.
Specifically, they often "request" this be done well before a term sheet. If the company complies then they start running out of cash, making them more dependent on the VC money and less able to negotiate terms.
Further, it's a mistake. These companies were using the income from other companies they were consulting for to build out the product. It's like free development. You get your engineer time paid for, and make a profit on it, and much of the work goes into the product side of things. All you need is to take some of those profits and build up a separate team that is focused on productizing the core product.
IF you do it right, you can bootstrap and never need to take VC funding. Either way, a successful consulting business extends your runway.
The claim about lacking focus is BS. Because the consulting- if being done right-- is in the exact area where the product is being built.
You know what takes focus away? Running the senior management team all over the country or the valley talking to VCs who are mostly going to waste their time because they don't have the balls to say "no"... looking for the 1 in 100 that will invest in the company.
I've seen this many times.
If you're able to secure consulting for your company and it's in the core area of what you want the company to do long term, do it.