If the question pertains to descending from a high MDA to TPA, which is below the MDA, before you are in a normal descent to land, then I'll have to disagree with that technique.
The main reason for a MDA is to guarantee you obstacle clearance during a non precision approach. I've been trying to remember for the life of me on how much obstacle clearance a NDB or VOR approach guarantees you, but I can't remember. Maybe one of you two will know.
Anyway, let's say that you are doing a non-precision approach to an airport that you are not familiar with. The weather is a little higher then the minimums, but visibility isn't the best. For this instance, the MDA is 2000 ft msl (1200 ft AGL), and the traffic pattern is 1600 ft msl (800 ft AGL). For this situation the MDA for the approach is higher then the TPA. Now, you start the approach and start descending down to your MDA. You are around 3 miles from the airport when you break out at your MDA. You see the airport and determine that you meet all the criteria in 91.175, so you decided to descend down to TPA to set up for a normal landing. You are 2 miles from the airport at TPA when all of a sudden your right wing makes contact with a construction tower.
By descending below the MDA, you were not guaranteed your obstacle clearance. I always thought that the TPA would assure you obstacle clearance when you are within 1/2 from the airport. That's why whenever you do a NPA, you always calculate a VDP even if the approach doesn't have one, so you're able to determine if you're able to make a normal descent to land or not.
As for circling approaches, I've never like those. They only guarantee you 300 ft obstacle clearance WHEN you are within the prescribed range for your aircraft category. Remember, the FAA does not take into account density altitude, and increase in true airspeed during circling approaches for their distances like ICAO does. There have been plenty of accidents when aircraft have struck obstacles on circling approaches because the aircraft had traveled outside the protected zones due to their turn radius increasing from higher density altitudes.