The only way that a landlord is 'disadvantaged' is that the tenant could give notice at any time during this year or the next 5 years and you have no recourse but to let them out of the lease.
Tenants do these types of things because they sometimes 'perceive' that this is a better deal for them in the event that they want to move or find something to purchase during the year, they are no longer locked into a 1 year term.
A Month to Month lease can often be a far better situation for the landlord because should you ever wish to move back into the property you do not have to wait until the end of the lease and only have to give 90 days notice and then tenant has to leave, by law. Or, at anytime you can now put the property up for sale and sell with a 90 day closing and again you do not have to wait until the end of the lease and the tenant has to move out, by law.
You are also in the same position as increasing the rent.
The Residential Tenancy Act is more in favour of the tenant and the tenants rights and security of tenancy is one of the key's to the Act. This means that when a tenant has a lease, they are secure in that lease, no matter what (except for major renovations and a few other unusual situations) and can stay in the property at least until the agreed to end of lease.
I have some information on my site:
You may browse to the government site to find http://www.ltb.gov.on.ca/
and shows all the forms and information regarding the law for landlords and tenants
Please let me know if you have any other questions or require further information.