The designer should consider several issues involving threaded connections. Each thread is an incipient crack in the pipe wall. Each time he stops the weld and restarts it he has another opportunity to undercut the weld. Seal welds on threaded joints are specifically disallowed by some corporations as a matter of policy, but not by the Codes. In the case of seal welds, it is understandable that there have been bad experiences because many of them are done badly - not good workmanship.
Bad welds CAN do more harm than good sometimes.
The B31 Piping Codes, however do not disallow seal welds - but have tried to point out some of their limitations and pitfalls. It can be seen that seal welds, per se, are only used in the B31 piping Codes with threaded joints. Welder qualification is addressed by paragraph 328.2. If seal welding is done, paragraph 331.1.3 should be reviewed to determine if heat treating is required. Paragraph 335.3.3 mandates that seal welds shall be made with no thread compound.