two reasons to use a separate account. one is what happens if you are ever audited by the IRS. it usually easier to satisfy an agent if there's a separate a/c that all the income has been reported and only rental expenses have been deducted. if your personal and rental items are all run through the same a/c an agent could start nitpicking on items. the second is that it will simplify the work you have to do in reporting the activity on schedule E. no having to look at a bill to see if is rental or personal.
as to LLC's some taxpayers put each separate property into its own LLC. what legal protection this provides varies from state to state.
