In general, I couldn't care less if you use 40/25 or just 25. Using 40/25 isn't a bad thing, but I don't think it adds anything or solves a major problem. In 17 years, I have had ONE crew that had a significant pace of play problem. Otherwise, all of them are within a reasonable range.
But here's where doing it without having play clocks does make a difference. We will sometimes (often) try to bleed the clock. On fields without play clocks, I stand next to our play caller (coach) with a stopwatch and let him know when we are down to a set point on the play clock, be it 10 or 15 seconds, whichever mode we are using. He then drops his hand and the QB can proceed. My watch is accurate because I watch the R for the RFP.
If we go to 40/25, I don't know when the FJ starts his 40 second count. I know it's supposed to be as soon as the previous play is whistled dead. But I also know from NCAA games that there is sometimes a delay in starting that clock, and that's when it's being done by a timer that is off the field. If the FJ is involved with the end of the play, breaking up players, retrieving a ball, etc, he may not get that clock started as soon as the play ends. Now, I don't know when he has started it, and we can't effectively bleed the clock.