Every tool has a function, a place and time to use them. Anything that has abrasive qualities will grind M.I.G. welds. As stated before, Grinding large areas with the belt sander is Belt use intensive and cost prohibitive at best.
I prefer the disk type sanders For large areas and carborundum type disk grinders for high build up areas The belt type sanders come in handy for hard to reach areas and tight spaces where the larger sander/grinders won`t fit.
If you don`t have one, you will find a 1/4 angel grinder/sander with attachments {Both, carborundum and abrasive disk} quite useful and inexpensive to operate.
If a man is going to weld he can`t have too many sander/grinders of different sizes and shapes in his arsenal of tools.
I even use the drum sander attachment with my dremel tool at times.
For real tight places a burr cutter can be just the ticket!
Dremel has a nice assortment of these items
https://www.widgetsupply.com/page/WS/CTGY/dremel-burr-cutter