Well, also with multi site there is the easier ability to share data across sites should you wish. Perhaps it depends more on your sites. Eg: dogs4sale.com, cats4sale.com, birds4sale.com and rabbitts4sale.com might make a smart multi site decision for cross promotion, sharing user logins etc. whereby dogs4sale.com, hotrodcars.com and vacationinmadrid.com might not.
Just a thought!