Won't repeat everything but; There are
a main set of combat abilities I use all the time, not including all of my different heals, but I did train all of my skills to 50 and use every single one of them. Some more than others ofc, some may work better in certain situations than in others and a couple are kept for emergency like cloaking screen (exits combat and puts me back in stealth) but each has its purpose (sometimes multiple) and then there's the different cooldowns, some are instant and cost no energy, some may grant me TA etc.
So yeah, I know I wouldn't be able to play my agent as efficiently if I didn't take full advantage & utilise everything I have available. From a solo pve standpoint I know I would struggle taking on some high level elites if I didn't have access to a lot of those skills because I'd have limited options available to me or they'd take me longer to kill. Group content I would have less abilities available than I should have which would likely make the content harder for the rest of my party too. - Skills like sleep dart, whilst I use this in both solo pve and pvp too something like this is even more vital in group content when you're taking on several elites. Or flash bang which will take that mob out of play for 8 seconds and give me chance to heal up. If I had never bought some of these..
Then there's pvp. Whether that's in a warzone or in the gameworld if I only had a limited amount of abilities going up against another agent in Hutball or a smuggler or whatever they would have the advantage on me from the get go.
The good thing about MMOs like this is that they don't give you too many new abilities at the same time, excluding higher ranks of existing that is. So when you get given something new to use you get time to find ways of getting the most out of that particular skill and seeing the different situations you can use it in where it may have a different effect. Like my corrosive dart. It's a DoT which I use it all the time on mobs to tick away whilst my companion works it down and I either keep her up or do some dps too or both. When grouped I can help do a bit of damage on the side whilst I'm focusing on healing as every little helps and in pvp I can use it as a finisher for someone trying to run away at low health or If I spot an enemy stealthed nearby me I can quickly fire the dart at him and as soon as that DoT starts ticking he's brought out of stealth and into combat. Also if an enemy is trying to capture our turret when I'm busy trying to keep my team alive rather than go off and start attacking him myself or have my team break off I can simply use the dart on him. Because it deals damage over time every time he tries to capture that point whilst the DoT is still on him it'll interrupt his action. So many uses for just that 1 ability.
annnd then when I get another new skill to use I can start finding effective ways of using that one then too.
Whatever works best for you at the end of the day I suppose mate but I did find it very strange and do think you'll be better off with everything your trainer has available for you