Welding leaves brittle heat-affected zones that are subject to fatigue failures and cracking without post-fabrication heat treating/stress relieving. The "screw and glue" construction is designed to overcome this without relying on the pull-out strength of the fasteners to provide structural integrity. In effect, the fasteners are there to provide clamping force until the adhesive sets up.
If you don't trust mechanical fastener and adhesive construction with aluminum, I'd suggest that you not get onto any modern commercial airplanes. :o
Rusty