Actually, i can answer this.
No one class will be gimped. There are certain abilities that are natural to one class. Those abilities will always be strongest on that class.
I could play the game as a marauder the entire game, and still be worth having in a party. I won't be as good as other marauders that have healing spells, and other abilities, but i won't be so gimp that you wouldn't want me in a party.
Seems more like speculation than anything. I'm not saying you're incorrect, but the impact your secondary abilities make, as well as how much min/maxing is needed to complete PvE content, has yet to be determined as far as I'm concerned. If they're so ineffective to the point where they make little overall impact, the whole customization system seems quite lame.
This is why I'm not a huge fan of ability freedom in MMOs. It's hard to balance, for one. If you want to allow people to play anything they want, then you must balance content based on the lowest common denominator. If you don't do that, then you end up with something like FFXI where certain job combinations are considered gimp and useless.
What you are arguing is that the lowest common denominator (aka someone with little to no secondary abilities) will still be able to complete content. This runs the risk of making content for min/maxers too easy, depending on how much of an impact those abilities make. If they make little impact, that's certainly not sounding too great for one of their more unique selling points.
I can tell you right now that on my PUG I have the Trunksplitter ability from MAR.
At level 9 PUG I can already do 200 more damage with that ability than without. Soloing is much easier.
So I have, Concussion Blow and Seismic Shock AND Trunksplitter.
Preferably, in groups maybe (I really don't know yet) I'd want only abilities that ONLY hit single mobs, so I'd get rid of Seismic Shock and want to level another class that had another single mob ability.
Or maybe I'd want all AoE skills, the same would apply.
The point being: I'm going to want abilities that will give me the most damage for the most TP.
So the hardcore are going to get the groups (or at least be more discriminating) because they'll have (want) the most abilities and will be the most preferred. At least in terms of DD.
But this is just how I see it, very early on in beta.