While I can not guarantee that your standard joint would not last long, we can probably agree on largely that these bushings were never ment to flex, or little more than stock. Correct me if I'm wrong, but radius arm uppers don't flex, or at least no more than stock. Most 4 links have jjs/heims on the body side reducing axle side flex. Three link is a minister of silly walks and largely has a modified upper axle mount(JJ or such) which will handle the stress of being the only upper arm, just as slowxj has explained.
I get what your thinking, heck I never thought of it until I started looking into 3 links, but a new bushing and mount is advisable. This does not mean a rubber bushing will not work, it simply begs the question for how long?
I think on the same token a 3 link may beg for a goid track bar system to?
I'll try to re dig up the thread I found some great knowledge in, a vendor explained much of the engineering. Ring any bells folks?