With the limits, people have to choose "the best/most common/most expected" weapon skills to have.
Pretty much this. I have 5/5 Res and 5/5 Shoha, I have to demerit Req now to get 5/5 SD. I would love to have Ruin / Upheaval / Req / Exten / Apex / Entropy to play with and be more well rounded but due to the need for maximum damage my three choices are forced.