The odds are probably in your favor, but when a weld lets go under load, it lets go. The heat effected zone around it can be more brittle than the parent metal, and I wouldn't plan on it giving much warning. And if you are lucky (or unlucky) enough to see a crack starting, now you have a broken frame to repair, likely away from home, which would likely require more than the two plates. Like you probably feel, I think it's highly unlikely that MY trailer will fail (the "it will never happen to me" notions that we all get), but someone's did. I'm going to have the plates installed.