The practical advantages of the union held together by the U.S. Constitution include a reduction of factions, proactive promotion of trade and wealth, and a more cost-effective government. In theory, as well as in practice, the new plan of government is far superior to the old and more likely to be sustained.
A republican form of government provides the closest remedy for factions without eliminating liberty altogether. A faction is defined as any number of citizens that are inspired by some common passions to act adversely towards the rights of other citizens. The republican form of government works to prevent factions because a higher number of representatives guard against the attempts of the few, and because the extended sphere of the republic makes it less probably that a faction will become a majority of the whole.
The union serves as a better means to promote commerce, especially in competition with Europe. It allows for uniform prohibitory trade regulations that would eventually lead to privileged trading in the British markets. It also provides for a federal navy that will assist the United States in establishing commercial privileges in this hemisphere.
The union serves as the best means of promoting the overall wealth of the young nation. Revenue in the whole nation will benefit from the consistency of commerce brought about by the union. The ability of the citizens to pay taxes is proportional to the overall amount of wealth, and this increases with commercial success as well. If there is no union, it will be easy for states to trade illicitly amongst themselves and smuggle contraband, both undermining the national revenue. With a single union, there's only one border to protect for trade violations--the Atlantic side.
If there is no national revenue, then taxes will not be taken from commerce and instead will be placed on the land. Landowners will become the most burdened class in society.
It is more cost-efficient to run a union government than to support 13 separate governments. It will be more expensive to staff the separate governments, especially because each would have to worry about inter-state threats to their own security and defense.