Why would they permit agents with different values?

Agents may not fuse together for the same reason that companies today do not: they are prevented from doing so by a monopolies commission that exists to preserve diversity and prevent a monoculture. In which case, they'll have to trade with and delegate to other agents to get what they want.

If you're implicitly thinking in some Hansonian upload model [...]

That doesn't sound like me: Tim Tyler: Against whole brain emulation.