It makes it very hard, sometimes impossible to upgrade a protocol, because some peer somewhere will try to interpret your new thing if it were a broken old thing; so security holes get enshrined. It is much better to fail fast, and make it easy to see why. The only people who benefit from enabling crap implementations, in the end, are makers of crap implementations. Their stuff should fail as soon as possible, so they have to fix it.

In some cases, it is acceptable have a "permit crap" mode that can be turned on in cases where the other end is not negotiable. But ideally the crap fails to work the first time it is plugged in, and returned to the manufacturer. Stuff that causes no end of trouble gets replaced eventually, but somebody has to expose the trouble, and it is best to have lots of company so the culprit is blamed.