I like it on paper. In practice, it's tricky to use.
The slice of the color pie includes having benefits for small agendas. It has since the beginning: see Archer. Archer gets significantly worse if you're pitching 2- or 3-pt agendas. We see that mechanic again in cards like Corporate Town, and fuel for agenda-forfeiting in things like Public Support.
The inherent drawbacks of having lots of small agendas are:
- Your agenda density goes way up, so the runner is more likely to steal agendas from central servers
- Your advancement-cost-to-point ratio goes way up
There are probably not a lot of great ways to offset #1. The Board was designed to help with that, but trashing it gives the runner an extra 2 pts and can pretty much lose the game for you.
As for #2, I'm saying it costs more money to score out with a lot of 3-for-1's than a few 5-for-3's.
In a PE deck, you might have 14 agendas: eight 1-pt agendas and six 2-pt, for example. If you were to score out (you should try, if only to pressure the runner), it might look like 3x House of Knives, 1x Chronos Project, 1x Clone Retirement, and 1x Philotic Entanglement. That's 17 (Yes, there could be other agendas like Fetals and Medicals involved, but I think this example still illustrates my point).
It's also why rush decks love Hostile Takeover - that net 5 gain helps to offset the additional advancement cost.
That's where Stock Buy-Back comes into play - it helps to offset that drawback of having lots of small agendas when constructing your deck. 2 stolen agendas (i.e. 2-4 points) nets 5, which is not bad. 3 stolen agendas (i.e. 3-6 points) nets 8 for a , which is awesome.
Easier Said than Done
In practice, SBB is hard to use because the runner needs to have stolen some points to be worthwhile. You don't want to see it in your opening hand. What if you see 2 of them early? Do you just leave out some agendas for them to steal so your Stocks rise? It feels like a Diversified Portfolio or Peak Efficiency in that it's worse than Hedge Fund early and better than it later. So I think SBB is actually pretty specialized.
I would try this in an Argus rush deck with at least ~14 agendas and 3 Snare!s. Don't ice up R&D early while you focus on scoring out some points, and the 2-3 agendas they steal in the meantime will actually boost your net worth. Then you sell your stocks and use that money to Midseasons them before blowing their shit up.