That's why index funds are so popular. Because they are cheap and still return as much as actively managed ones, on average. TSP doesn't have any redemption fees or fees for reallocating, so there's really nothing stopping investors from tryng to time the market like that. But, don't let that tempt you. Just think, the experts are analyzing companies' financial statements, ratios, price history, economics, everything, and they still can't beat it, (like the Bird said). So, if an average Joe like us were to somehow win, it would be complete luck. Just expect around a 10 or 11% stock market return (C fund in TSP) over long term.
I'm in the military and there is no matching for the TSP. I stopped contributing because of that. Just no incentive, as I can get into a cheaper traditional IRA from, say, Vanguard. Plus, I can't check my TSP prices from WSJ online, so it's a bit more work to go to their website. I'm getting out soon, and will transfer it over to Vanguard for easier recordkeeping and saving on expenses.
If you get an employer match, then TSP would be worth it, IMO.