I think the 2 benefits from doing 6 issue miniseries are as such:
1: New #1 every year
2: Easier to put into a trade.

I prefer this IDW stuff to DW's work, the art is not so vibrant, but it has a soul, something the DW stuff severely lacked.

Interest is being lost due to the stop start nature of the series, and all the excess lines (G1 reprints, Evolutions, Movie stuff, Beast Wars), even Spotlights are very hit and miss for casual fans.

However I think IDW know they are mainly catering to a niche market, an d as such are relying on them to hold up the franchise until bigger events (like the All Hail Megatron and Movie Sequel comics) bring more casual readers to the fore.

Having Furman as their mainline writer is a double edged sword, having the one writer writing everything means there is more cohesion in the IDW Furmanverse, on the other hand he is a very popular writer nad must spread his time across multiple frnachises, meaning the 6 month or so gap between the main book titles is almost necessary for him to do his other works.

As long as IDW do not start royally stuffing up their release schedule (like tehy did with the BW stuff) I think they will survive. They may struggle time to time, but they will survive.