As far as I'm aware, your FeeShares generate ShareCoin interest just as holding ShareCoin does. So 1 FeeShare will generate 6,000 ShareCoin's in the first year (potentially 12,000 if they generate 60% in year one because they are held in the exchange). You will also get 0.001% of SharexCoin's profits paid to you in BTC.
So the downside of buying FeeShares is that you can't sell them (at least not yet), so you can't dump the 20,000 coins you put in. It basically forces you to hold your coins.
The upside is that you get the same interest as everyone else holding 20,000 ShareCoins in the exchange, but you also get a share of the exchange's profits.
If FeeShares we own do become tradable, then there will be no reason not to purchase FeeShares as you will get the same benefits as owning ShareCoins but also a share of the exchange profits. Well actually, the only downside would be if the ShareCoin value changed at a different rate to FeeShare value.
Right... also, you can always sell the SHARE that you get for interest. So, lets say you buy a FeeShare. Year 1 you will get 30% interest on that. You get that interest every day. So, 20,000 * 30% = 6000 SHARE * 250 SAT (current price) is .015 BTC (if I did my math right) a year. Plus the .001% of the Exchange's revenue. Granted not a l lot. But, if you were given 3 FeeShares you have 0 risk. Also, there is a potential that SHARE price will go up... which means your SHARE interest is worth more and the Exchange profit is more. This doesn't count revenue from the Gaming site which is in the plan. (although I'm not 100% sure the exchange FeeShare's apply to that)
Or you could hold your SHARE in the exchange and get 60% interest the first year.
Or, sell it now for like $50 - $60 worth of BTC.