I think that is a fair point, particularly if there is direct sun. I do think that even temps in the high 60s can be manageable with a low dew point for those who are heat adapted.
I went back and looked at Boston this year which was pretty tough, and the starting conditions were what the finishing conditions are forecasted to be for the 3:30 marathoners (I chose that time since that's my wife's goal time). I have to admit that it was uncomfortably warm in Boston. But I do think that the time adjustments estimates were just about right on in my case, by my best estimate.
I agree with this. If you've ever run in 70 degree temperatures with dew points in the 30's, you'll feel like you're running in great conditions. Granted, 70 with 30 dew point is pretty rare, but Smoove makes a very good point. Dew point to me is almost more important than temperature.
I'll add that the MPR calculation for me is spot on, I use it for all my runs in the summer and it's usually within a few seconds of what I actually run.