Currently when I have a welder and I see a marine that could use repairs on his armor, I ignore him. Its too risky to switch to a welder, since you never know when you need to switch back in case of attack.
I liked it when you simply had to go up to a marine and press the use button. I would repair marines much often, since it was a quicker, less risky transaction.
I propose that if a marine has a welder, he simply needs to press the use button to repair marines and structures that need it. It won't interfere with building structures since they are two different things that don't overlap. I think the welder also needs to have its range increased a bit, since it seems im always just barely out of range while following a marine that needs repairs.
Marines can still equip the welder like now in case they are mass repairing multiple buildings (don't want to press the use button the whole time) or damaging alien structures. It would be nice if you need repairs, and there is marine with a welder equipped in his hands, that an icon would appear over his head indicating he can repair you.