[M4IF Technotes] Constraint on vop_time_increment_resolution obeyed?

[Kris Huber]

I noticed the following constraint in the text of the MPEG-4 video standard.
Near the end of clause 7.7.2.2 about Motion vector decoding in B-VOPs:
"When interlaced direct mode is used, vop_time_increment_resolution must be
the smallest integer greater than or equal to the number of frames per
second."
This affects the VOL header.  What surprises me is that it seems to exclude
the possibility of exactly indicating the time of the source fields, which
come at a rate of 1001/60000 = 29.97 Hz * 2.  It also seems to impose a
quite coarse quantization of the presentation time indicated in the
bitstream.  For the vast majority of video source available today that's not
a problem, but perhaps future devices won't all use the same fixed rates.
Please comment.  Is there a reason to obey this constraint?  Without doing a
lot of detailed searching and thinking, I suspect the constraint could be
there to simplify direct-mode interpolations or just to save a very small
number of bits for the representation of the time.
Thanks,
Kris