Leaving aside the question of round trip versus one-way fares, there are some potential disadvantages to booking your outbound and return flights on separate one-way tickets as opposed to a single round-trip (aka return) ticket— unless all tickets happen to be fully flexible, fully refundable, full-fare tickets, which you won't be buying if you care at all about price.
Using separate tickets for each leg is not unlike booking connections on different tickets— you assume all risk if there is a problem with one leg that prevents you from boarding the next. If you find you need to fly into and out of PBI instead of MIA, you face two change fees instead of just one. If your outbound to OXR is canceled and you get rebooked to LAX for free, you'd need to change your return in a separate call and pay a change fee with the second airline.
Furthermore, if your first flights gets canceled or you get bumped and they can't reschedule you until after your trip becomes pointless, you won't be able to claim a trip-in-vain refund. And while you might be able to find a sympathetic agent for adjusting two separate one-ways on the same airline, it would be most extraordinary to find an agent sympathetic to changes made by another airline. (Of course, the longer the time between your flights, the less the above is a serious concern.)
Secondarily, you may lose some advantages built into round trips, though this is largely dependent on the particular fare rules of the tickets you do buy. For example, a round trip award ticket on United Airlines allows a stopover or an open jaw in each direction, but no stopovers are allowed on one-way tickets, severely reducing the benefit for travelers like me. Additionally, some coupons and other promotions may require a round trip fare.