As your doctor likely discussed with you following surgery during the discussion about the treatment options available for the wound, many wounds will heal if left alone. Most wounds left to heal on their own heal will do so with worse scarring than if they are surgically closed immediately - however the forehead is one of the few areas that does tend to do ok and in some cases does brilliantly. The longer a wound remains open, typically, the worse the scarring. We know that wounds that have not healed by 21 days tend on average to heal with thicker scarring than those that heal before that (we know this from the burn literature). Smoking, nutrition and general health all affect wound healing.
In addition to an unpredictable cosmetic outcome, secondary intent healing is limited by inconvenience to the patient (long time to heal); infection is very, very rare in such wounds on the face.
Most of the doctors who perform the Mohs procedure are dermatologists and many do great work. There are seven of us worldwide (myself included) who are formally trained in Mohs surgery and are also board certified plastic surgeons; the advantage is the highest cure rate from Mohs, followed by immediate cosmetic reconstruction. I prefer to close wounds since I feel the result is predictably better when I surgically limit the number of factors/hurdles I am asking the wound to overcome.
That being said, I wasn't there for your cancer removal so I can't comment on the depth or what your doctor had in mind. I would trust your doctor (you chose to go to them in the first place) and know they have a plan in mind to ensure the wound heals in an optimal cosmetic fashion; I would certainly encourage you to contact them with any apprehension or concerns that you have; I would give them the benefit of a doubt that they would want to know of your thoughts and concerns and it would likely help you both to discuss it.
An unsightly scar after secondary intent can typically be addressed down the road to make it look better as well and a suboptimal result doesn't mean anything was done wrong - sometimes it takes a few steps to get to the final result. The wound healing cascade takes 9-12 months during which scars tend to soften, lighten and become flatter and more supple so patience is also a virtue.