This is due to the nature of how skill trees work with level expansions.
When you give more skill points to spend to a player, you also make it possible for them to buy more combinations of the lowest level branches of multiple skill trees. They can, for example, buy the lowest level keystone abilities for both a strong DPS character AND a tank, and make characters that are virtually impossible to balance correctly (the classic 'magetank').
We're not thrilled with this, of course. We'd really like for tank classes to be able to tank as quickly as possible. However, some things would need to be rethought before this can be a reality. It's definitely something that will get more thought before we ever raise the level cap again.
This is sensible reasoning, of course, but it does require that the capstone ability be worth the extra expense. What I've found since 2.0 is that some trees (especially for PvP) just don't need their capstone more than they need the flexibility of going hybrid. For example, with the changes (nerfs) to Assault Plastique/Thermal Detonator, I have found it better for my Vanguard to just take damage boosts from early in the tanking and other DPS tree. It works, but it's ultimately unsatisfying. Getting that capstone should feel worth it. Likewise, the capstone should be important, but not required to make the spec work. Examples of that IMO are Sawbones for Scoundrels, the Shadow Tanking tree, and others.
One of the best trees in my opinion is Arsenal/Gunnery. Heatseeker/Demo Round is an awesome ability, but you don't necessarily feel useless until you get it with the boosts to unload, rail shot and tracer missile.