Firstly, the reason against it are accessibility and balance issues. Skill trees make balance much more challenging and tend to make build effective characters more difficult. Almost always you end up requiring third party sites to be sure your build is actually any good.

Secondly, there are quite a few jobs that wouldn't really work in multiple roles. BLM for example makes no sense as a healer or a tank. Lorewise they are the literal glass cannon. Likewise, SCH wouldn't really work as a dps cause it would be too similar to SMN or Tank because its a guy with a book.

Finally, Yoshi P has talked about this before and has said rather than try to make two builds for a single Job he would prefer to just add another Job with its own themes and Aesthetics as the bulk of the work making a Job vs making two versions of the same Job wouldn't be that much different.