Center speaker OR Full range as Center speaker?

My experience has been that, ideally, 3 matching across the front is best. That however needs to be quantified in the context of the system/room/setup. Basically, a lot of full range speakers, or mini's for that matter, aren't really all that capable of producing solid, dynamic, coherent, and focused sound, period!...inlarge, they make ineffective movie speakers IMO. And I've sold alot of audoiphile speakers over the years.
There is something to be said for a good speaker for movies vs. just for music. Dual midrange/bass woofers, or others.
Having reinforced midrange, bass and treble drivers, either using horns, or dual/multiple mid/bass drivers, multiple tweeters, large pannels, and/or high efficiency designs in general(like active's, horns, multiple driver array's, etc) often make for a more solid home theater speaker. This is especially true in the critical center channel, where a lot of action is mixed!
Basically, there are a number of designs that are just to week, laid back, uninvolving, "open", and polite sounding do to true justice to what I call "the hard stuff"!
But, there are considerations.
If you have to use a traditional designed stereo speaker for HT dubties, they most always work much better not only crossed over as "small", but used in a smaller setting, or where you sit acoustically "closer" to the speakers...thus you hear more dirrect sound vs. reflected sound from the room boundaries. This tends to "reinforce" the sound from the speakers more for your ears.
Still, all things equal, I'd say it depends greatly on the design of the speakers.
Also, since most typical full range consumer home audio speakers are passive, and low on efficiency/sensitivity, it often makes for better control and powerhandling when you cross over the speakers as "small", and dictate the more demanding dynamic bass info for an "active" speaker, like a powered subwoofer.
Now I have head some full rangers that do pretty darn well as "full range", and as center channels. Big Dunlavy's, Avantgard horns, PSB Stratus Gold's, Infinity Prelude MTS's,NHT VT3's(powered subs) and other powered models, maybe a wilson WATT PUPPY, and other larger, higher efficiency designs.
Ok, this is all my experience, and you can really do what you want. I just think some designs don't do it so well.

Krell man and Kr4, I concure and relate to your experiences entirely. I've sold Dunlavy in the past, as well as Mirage. You both understand what's happening by-enlarge there.
The Dunlavy using multiple woofers in a coherent time aligned array definitely reinforces the frequencies, and proves very fast, coherent, and solid in imaging, not to mention great dialog inteligibility. Thus my findings that these designs most often have an advantage over single driver designs vs, say Dappolito, Horns, THX, plannars(sometimes), etc. Also, active help. Nonetheless, the Dunlavy's indeed make a potent enough sound for movie mixes I've found. There's a lot to be said for that design.
Still Krell Man, may I strongly suggest crossing over the Dunlavy's at least at 65hz or 80hz, over to powered sub(s)!?
I could easily bottom out and distort even that efficient design SCIV speaker in my systems. And I found better to cross over to "active woofers". That was my finding anyway...which is consitant with other lesser efficient designs as well IME.
Anyway, I think you'll get better dynamic range and capabilities ultimiately that way. Some will find the bass and volume levels they chose adequate full range however with the SCIV's, and that's fine of course. Still, at closer to THX levels, you will reach limitations.
Would I use Dunlavy's like this for HT if needed? I would indeed. My only problem with Dunlavy's now-a-day's is matching drivers when they go south(if). That was the design of that speaker, matching tolerance drivers.