>Is it merely a matter of weight when loaded or are there other reasons?
Weight and interchangeability.
http://www.bushfarms.com/beeslazy.htm#uniformframesizehttp://www.bushfarms.com/beeslazy.htm#lighterboxes> My reason for asking is that I still have another hive to build for this year and was planning on getting a hive kit from Mann Lake. (this was the supplier that supplied the hive that was given to me) As you all likley know, they all build their kits with two deeps and two mediums. Is this an inefficient or ill-advised configuration?
IMO, yes. It's not interchangable (deep frames don't fit in medium boxes and medium frames leave too much room in a deep box) and it's too heavy when full of honey (a deep full of honey weighs 90 pounds).
> I had anticipated keeping the plastic foundation in the deeps and then using starter strips or an angled top bar as Michael Bush has recommended in the honey supers so that I could cut comb or crush and strain easily. I am starting to rethink that strategy and now wonder if I should omit the frames in the brood boxes as well.
Not sure what you mean by "omit the frames", but you have to have frames and some kind of comb guide or you'll have a mess. If you want natural cell size or small cell size, then that's an issue. But if you don't care about cell size, the plastic works fine.
> I already have two hives in the 2 deep/2 medium set up. I do not want to build a third if it is an incorrect method.
It's not an "incorrect" method. It's actually the most common method. It's just an awkward one. If you intend to convert to all mediums it's easier to do now than later. I found having all the same equipment so important I cut down all of my deeps to mediums and all of my ten frame to eight frame. It was a lot more work to convert but was worth it to me.
http://www.bushfarms.com/beeseightframemedium.htm