Yes, let's explain a bit more.
1) First, regarding the mirroring question, let's be clear: the 3 "trees" of perks for an advanced professions are exactly mirrored, with other names, for the equivalent profession of the other side: for example the 3 trees offered to a Jedi Counselor Sage are the same as the 3 trees offered to a Sith Inquisitor Sorcerer. Only the names change.
2) Now, how does a profession (class) work?
Your first pick a main class.
Then at level 10 you pick an advanced class.
See my first post.
(You can wait and pick the advanced class later but that doesn't make sense to wait more than one level or so.)
At this time, your are offered 3 perks trees for the advanced class you picked. (The system used by SWTOR is extremely similar to the one proposed by Age of Conan.)
For example, a Jedi Sage will have the trees: Seer, Telekinetics, Balance.
Note that the names used for the trees are more or less misleading or don't make necessarily sense.
You have 41 points to use in those trees: you gain 1 at each level, from level 10 (included) to level 50.
Each tree has 7 "floors". To be able to train a perk on next floor, you must have 5 poins invested in the preceding floor, so you can't just pick the perks you want.
Each tree, if fully checked, uses -it seems- between 36 to 38 points (not the same).
This means that, if you fill up a tree, you have almost nothing left.
That said, the trees can be very confusing and you will understand the interest (or lack of interest) of some perks only when you are at a high level and got some more abilities, and experimented with them.
For example, in the "Seer" tree, you have, on the second floor, a perk increasing the damage (or heal) of area effects (AoE) by 5%, and even by 10% if you invest 2 points in this perk. This perk can be trained as soon as level 16 (6 points unlocked = 5 for the first floor and 1 in the perk) BUT, actually, at level 16, you do NOT have any AoE! Worse: you will never get a lot of AoE's... so what?! So... you will get at level 34 "Force Quake" which is a very powerful AoE (damage, slow speed, knockdown) and boosting it by 10% can be really interesting it seems but you can't really know this when the boosting perks becomes available. (You also have, a bit before, an AoE "Force in Balance" but it's clearly less powerful so boosting it doesn't make as much sense.)
3) Customization and versatility:
Yes, to some degree, you can tailor a custom character for your gameplay, especially as having the top peek of tree is nice but not necessarily at all better than investing wisely in like 2/3 of a tree + smartly spent points in another tree.
Unfortunately the danger I see may come from other players that might ask, for example, for a sage "with full Seer tree". This is applicable to other professions as well.
And, in the case of a Jedi Counselor Sage, I'm afraid that training Seer to the max makes soloing very difficult for you. (Less damage and crowd control.)
As in most games, you should be able to define one "profile" for soloing and one for teaming...
