I can see your frustration with the fast turnaround between each UFC series, but I think the only reason they're doing that is due to the high demand for the product!
Round 1 got scarce really quickly ($600 box now!) & Round 2 is starting to dry up (Sold out on ufcstore.com).
I think if they waited another 6 months before Main Event was released, it would have been better for the market (let alone Series 4).
On the flipside I think each series is getting notably better in terms of the quality of cards (see Main Event compared to Round 1), and Series 4 is looking pretty awesome imo...