By all means, plug weld it. If you don't have a welder, find a shop you can drive it to, and have them plug weld it for you. A real body guy could plug weld it for you in less than an hour, well prolly a lot less, but they will charge you an hour of flag time. $35.00
I would not want to have a front end collision with that car without the core support properly welded back on the car. It is a structural part of the unibody/chassis, really, its more important than you might think! It ties both frame rail togather! Not only is it important for structural integrity, but everything attached to it, like radiator, ect. will be vibrating.
Me, bodyman, painter, bodyshop owner 30 years