Well, I did a few quick tests. As Duke John points out, there is definitely a morale bonus from units with the "command" attribute. Working out if there is a bonus to attack is a little more complicated.

One way to test at what level of bonus, in comparison to the General command stars, the "command" attribution gives would be to measure the radius-of-effect of the unit. But that would require the ability to measure distances accurately on the battlefield.

Does anyone know of a reliable way to measure distances on the battle map? There is a RomeShell command -- output_unit_positions -- but AFAIK that is not activated in 1.2.

Anyway, for comparison, I setup a test battle between two groups of peasants, with either a command unit or a general. To be honest, I didn't see much improvement in combat ability with the general on the field. From Jerome's rules of thumb, my 10 star general should have given my peasants +10 to attack. With that advantage, I would have expected them to mow through their counterparts. But the test battles were usually close, although my peasants usually won (although not always) with 75-80% losses, or thereabouts.

So I modded my peasant's attack to 11, and retested. The battles now went more as expected. The opposing peasants were all killed for the loss of only ~20-25% of my men, and there never seemed to be any doubt on the outcome. So there may be a problem with the General's melee bonus. Either that, or they've changed it for 1.2.