Depends a lot on how bad the oval is. Might last for awhile, but you'll have less camber and KPI on that side, which may make it steer funny. Might also cause steering wander, especially if the suspension isn't forced to the rear when the toe-in is set; or pulling under braking.
Personally, I wouldn't bother putting it back together that way. Those arms are probably not available new, but used ones should be easy to source. If there was no other choice, I'd at least try to fill in the oval with a mix of JB Weld and fiberglass.