Chain guards aren't any more "neccisary" than seat belts in your car, a smoke detector in your house, or having your blood pressure checked.
If in the course of everyday life, if you never have ANY problems, they're a waste of money. But who do you know that goes through life without ever having ANY problems?
You can take your chances and go without. Maybe you'll get away with it, maybe not. You can add rubber belts that PROBABLY will do the same job.
Home-made chain guards, if built correctly, do just as good of a job as OEM.
The intent of chains (vs rubber belts) is to allow an object to exit from under the mower, but in a controlled manner. Belts MAY keep the object under the mower longer, increasing to chances of breaking a blade, sending it flying.
Insurance companies consider chains the ultimate in protection. I own a mowing business like LoneCowboy. Most of my business is mowing property being managed by a court ordered conservator. By the nature of their business, protecting themselves legally is a prime consideration. They have specs for contractors mowing for them. They require chains. I've bid on a few state and federal contracts. The bid specs stipulate using chain guards.
Now if you're not going to be mowing under the same circumstances as I am, you can roll the dice and NOT use chains, but should you choose to operate in what is considered to be the safest manner possible (Insurance industry risk management studies) , they are the best way to go.