I always seen semi-dwarf trees a s a good compromise between size and maturity rate. Standard trees are bigger (produce more fruit) but take longer to mature/produce. Dwarf trees mature/produce pretty quickly, but they don't get real big (produce less fruit). I am sure there is much more to it than that, but that is how my simple mind sees it.
The thing I would check into would be to ensure the rootstock on those trees is suitable for your soil conditions. Just because they are "semi-dwarf" doesn't tell the entire story. That just tells you roughly how big they will get. There are different rootstocks to produce better on different soil types and the like. I would have to defer to the fruit guru's here to tell you what works better where.
The terms Standard, Semi-dwarf and Dwarf may not mean the same thing to everyone. A good example is last year when I called a well known nursery asking about the trees they advertised as "standard." I found out that these trees were actually on MM111 rootstock, which nearly all of us on this forum refer to as semi-dwarf. MM111 is a good rootstock for many situations, but it doesn't make what I call a standard tree.
Also, MM111, B118 and M7 are all generally referred to as semi-dwarf. The first two are excellent for many applications, but M7 has been described as a "noxious weed" by some experts. A good number of trees planted on M7 will likely fall over at some point in their lives unless they are supported in some way. I've been told that if you buy a Franklin Cider apple from Stark Brothers right now that you have no choice of the rootstock - and it could come on M7 - which I would consider horrible.
On pears, a good and commonly used rootstock for many soils is the OHXF 87. It gives a semi dwarf tree. A good full sized rootstock is OHXF 97. I could be happy with either of those.