For me at least, it's kinda hard to find TF comics in my local area and ordering stuff off eBay and then having to wait for it is kinda a pain, not to mention you have to buy a couple of items just to get some decent postage pricing. Contrast that to the 5 minutes it takes to buy and download comics on my iphone and they are a lot cheaper too (like $2 each).
Besides as Sky Shadow pointed out a lot of IDW's stuff simply isn't worth it, stuff like Ongoing and Heart of Darkness and the DOTM prequels, I don't see them as being worth going to all the effort, when I'll probably read them once and toss them in the collection.
Now the good stuff: LSotW, HoS, Devastation, Megatron Origins, AHM I have all in comic/TPB form, but I'm satisified using my phone for most of the rest. I do the same with the Marvel and DC apps as well.