We are planning to make an offer on a house, but learned that it has just been leased for a year. If we buy the property, can we just assume the lease? What are the pitfalls of an arrangement like this and does it give us more negotiating power? We are in no rush to move in as it is our second home.

Do you mean that the property is being leased to a tenant currently? If the owner has leased the property to a tenant, then you must honor that lease when you purchase the property. Its not a matter of assuming anything, it goes with the property. You can inform the tenant that you will not be renewing the lease when it expires and allow them to move out early if they prefer.

The pitfalls are that you have to honor the lease, but you will also get the rentals. But you will be a landlord with all the problems that can entail. If the tenants are good, then you don't have a problem, but they could damage the house, not pay rent, etc.