Large corporations should be allowed to merge. There should be precautions, such as blocking the deal if it will cause a monopoly. But, large corporations are not automatically bad. A larger corporation can bring economies of scale and potentially lower prices to the consumer, although this should be weighed against the potential creation of a monopoly.
Very large corporations should not be allowed to merge with each other, as this creates juggernauts of industry that can either buy up or put small operations out of business. Additionally, these large corporations can consider themselves "too big to fail" and take risks that put the economy, or even American citizens, at risk. Monsanto already has enormous power and sway in bending regulations in their favor; their merger with an even larger company is downright frightening to think about.
No, mergers of large corporations should not be allowed because they prevent small companies from competing. No one will be able to start a new business and compete with a merger like this. There is no hope for the little man when monopolies are formed, and the US should work against them.
Bayer recently bought out the infamous "BigAgra" company Monsanto. This only gives Bayer more power if they can also control Monsanto's holdings. Monsanto is already too well-known for "copyrighting" seeds, so who knows what nefarious things Bayer will do now that they have access to Monsanto's business model and strategy?