A lot will depend on the penetration. Steel tends to penetrate nicely and you can easily and safely profile the weld lines. Also it's not all that hard for a good welder to get the smooth fillet look in steel and with stainless steel.

However when welding thin walled aluminium I think you'll find that the welders or the machine setup folks don't go so much for full joint penetration. With joints of that sort the bead is needed to bridge the structural gap.

For example I've read of two stories where guys with japanese sportbikes sent in their aluminium frames for polishing. The polisher, being used to working with steel and castings more than welded assemblies, "helpfully" ground down the welding beads and then lovingly brought the metal to a mirror like finish. The frames were ruined in the process as cracks occured almost immediately when put back into use.