Would selling that ability to have 3 more slots available to you at lvl 49-50, mean you're characters have a bit more power? Of course! But not enough to really hamper anyone who ignored it. It's a bit of +enhancement to a power, and 3 additional set bonuses.
If they implemented it as a 5 dollar global unlock feature, then people wouldn't have to feel it's a waste of money for every character they want to min/max, and those who don't want to spend any money aren't really left out of anything.
I agree with this in theory, but what you do need to be careful of with this sort of thing is "power creep". Sure adding 3 more slots as a global unlock won't break the balance as long as that the only thing you add. The problem is that you then start getting the situation where people ask for other minor power boosts and eventually they do start making a significant difference. The devs would eventually need to set a line and say "no more bonuses beyond here" and setting the line at "no bonuses, period" is a lot easier than setting it once people have been able to purchase things like extra slots.
We've already seen something of the sort with the Veteran Rewards system and booster packs. Sure the powers themselves are minor but Vets do get a lot of them and it does make them more powerful (especially at lower levels). There's a reason the later Vet and Booster powers tend to be more convenience/travel powers than actual attacks.
If they implemented it as a 5 dollar global unlock feature, then people wouldn't have to feel it's a waste of money for every character they want to min/max, and those who don't want to spend any money aren't really left out of anything.
We've already seen something of the sort with the Veteran Rewards system and booster packs. Sure the powers themselves are minor but Vets do get a lot of them and it does make them more powerful (especially at lower levels). There's a reason the later Vet and Booster powers tend to be more convenience/travel powers than actual attacks.