The main reason to not use frame mode is to avoid the effects on motion - sort of a strob effect, caused by the 1/30 time between refreshes of the image. The 1/60 refresh rate of normal mode (for NTSC) gives a smoother character to motion. Ultimately go with what looks best to you or paying client!
This has been my main reason for not shooting in frame mode. I've tried the setting while setting up shots and was disturbed by the jerkiness I saw in my viewfinder. I always switch back to 60i for the take. Seems like I am always shooting for clients. Rarely get a chance to shoot for myself. Someday...