So in essence its better to have an individual skill at 5-6, as its the sweetspot and the best you can get out of a skill before it becomes costly. BUT certain skills provide a train skill to your BO`s and the only reason why one would max out a skill.
Sometimes, the tooltip will say, "Learning all 9 ranks of this skill will allow you to learn: "RANDOM SKILL""
For instance, learning all 9 ranks of Heavy Escort Captain will allow you learn Attack Pattern Beta III.
Read it again. You don't learn it. You are actually able to teach it. To any Bridge Officer of that discipline. If I'm understanding correctly, with that, someone can trade you a (given your example) Tactical BOff, and you can teach that skill to that BOff and trade it back.
While on Spacedock 1 (Earth Spacedock), you can train another one of your officers in a skill that you are able tot each (by getting 9 points into certain skills). You don't use it yourself, it teaches one of your BOFs how to use it.
For T1 and T2 skills, I'd say it isn't worth it to skill up higher than 7/9 because of diminishing returns, unless it affects something that you want to have as much as possible of (for example, I always max out Spaceship command because I want all the hull and turn radius I can get).
For T3 and higher skills, though, the diminishing returns isn't nearly as bad, and so maxing out a skill gives you a better benefit than it does at lower levels.