I too have seen some "pretty looking" welds only to find out when I UT'd them, they were pretty on the outside only. I feel full pen welds should be checked other than with just visual. On the other hand if visual is all that's called for in the owner's specs, then you get what you pay for.
I check 100% of all the full pen welds that go out of this shop, only to ensure our customers that when thier drawings called for full pen, that's what they got, good sound weld. We have another shop about 350 miles from our main shop. When I first started Inspecting at that shop, I had lots of full pen welds failing UT. They told me, "If I knew it was going to UT'd, I would have done a better job". So, my response was, that if the drawing called for full pen, then they should have treated it like it was going to be UT'd or RT'd and no short cuts. After these guys cut out all that weld (dozens of W14 x 500's and W14 x 550's column splices), they had very few welds out of hundreds, ever fail UT again. That shop became proud of thier welders and trust was gained in thier ability to produce good sound welds and that no one had to worry about them when thier fabrications got to the field, whether or not it would be UT'd. Going alittle further than asked, gives your customers more confidence in your shop and it does not take any more time to do it right the first time. That results in work being directed your way when times are lean or when you are not the lowest bidder, but they know you'll do a good job for them so they use you over the lowest bidder.
Sorry, I don't have the sympothy for the welder that has to fix his welds because he didn't pay attention and keep his welds clean and it failed UT. I think it makes him a better welder when he knows where he went wrong and next time there will be better results when his welds are checked, other than visual.
Sorry for the rambling's about our welders, but I was remembering what I was told the first time I heard, " I didn't know it was going to be UT'd".
John Wright