If you publish a paper with a coauthor sometimes you should receive half the credit because you may have done only half as much work. On the other hand if you publish the paper with Einstein sometimes you should receive more credit (than for a single author paper) since Einstein's name suggests the paper is more valuable. Even if the coauthor is just Joe Physicist it may be important that someone, anyone, agrees with your ideas. Each paper really must be evaluated on its own merit. No fixed weighting scale is as good.