While it is true that more colonies will increase trade opportunities it should be pointed out that it's difficult to saturate all the potential trade between even two colonies of ten billion. In my current game a planet of five billion produces over sixty thousand units of goods per year while freighters move ten, twenty or fifty units at once depending on their size. So if you have two colonies then you'll need hundreds of freighters to saturate just one route (it's oversimplification as there is also the matter of demand, not simply production, but the larger point still stands). To be honest, since you have several colonies of five billion or more, adding more planets will likely not result in more trade income, unless their positioning will make them shorten the trade routes, as the earnings depend only on the amount of cargo moved, not distance, so whether your freighter spent five days or five hundred days moving cargo, you'll get the same amount of money.
The population growth has already been covered. As far as I know there is no point where population growth as a percentage becomes constant, so more colonies will always result in larger population growth.
Other than that there really is no reason to spread out, unless you're role-playing.