Is the Preston Garvey impersonator good for anything specific?

No it doesn't provide any interesting dialogue

The Preston Garvey impersonator is just a random encounter - a scam artist attempting to make some quick caps by playing off the goodwill of the Minutemen.

The player has a few options:

• Hand over the requested 100 caps
• Refuse to hand over the caps
• Claim (truthfully or otherwise) to be the Minutemen General.
• Claim to know the real Preston Garvey.

Having been 'found out', the impersonator will attempt to flee from you if you choose the latter two options.

Disappointingly, the real Preston Garvey has no unique dialogue to offer before, during or after the encounter, even if you encounter the impersonator with Preston as your companion.

Some info from personal experience and some from the Fallout 4 Wiki