Why vertical progression is poorly implemented

[CyrilLucifer]

It doesn't matter at all since there is always a superior spec. Anything less than ideal will be kicked. Remember the original xi guide that state war/whm is a good combination?

Bad example, that guide was filled with a lot of flat out wrong information. No one in their right mind would use THF/BLM, one of the "recommended" combinations for battle. There is no point in it. XI's mechanics did not allow THF/BLM to work outside of traveling and running around in towns.

The XI guide was also a house of misinformation as well as wrong information. It also said that SMN/BLM was good for nukes, which was absolutely absurd because of the way skill levels work in XI. You can't even get an unresisted nuke in XI without the appropriate skill level, which is impossible to reach with /BLM. THF/BLM and SMN/BLM being bad have nothing to do with efficiency and everything to do with XI's skill mechanics, which XIV lacks.
Disclaimer: At 119, SMN/SCH can, however, nuke with Dark Arts with the way gear works now. It's actually really neat, some people use it in their builds.