The organizers of my local cross series just announce that they will be allowing mountain bikes in the cross races instead of having a separate MTB category. I don’t know how I feel about this because I thought the whole point of using ‘cross bikes was not that they were the best tool for the job so much as that the best bike handling skills were rewarded more when nobody had suspension forks and fat tires. I’m torn about whether I’ll even bother to use a cross bike as I suspect a hardtail 26” MTB might actually be faster on most cross courses. I’ve never ridden my MTB on a cross course to find out, but it kind of feels like it isn’t really cross racing if everyone isn’t on skinny tires and drop bars. It seems that MTBs will become the bikes of choice for these races and they will become more like short circuit XC mountain bike races rather than ‘cross races. Has anybody ever done cross races with mountain bikes? How did it work out?

of course you'll be faster on a MB duh, that's what's so great about cross, they use the wrong bike.

Well that's what I would have thought too, but I was hoping the race organizers knew something I didn't (such as a reason why anyone should use cross bikes at all if there are no drop bar & max tire width requirements). If Mountain bikes are obviously so much faster, then everyone will just use them and we wouldn't really be cyclocross racing. Personally I just do if for some fun, exercise in the fall, and to help improve my bike handling skills. The more I think about it, I'll probably just do the races on my SS cross bike because that was what I enjoyed racing last year, and since winning (although it would be nice) isn't my main priority. I can imagine, however, how I might feel if cyclocross were my schtick and I now had to try and compete with mountain bikes. On the other hand, I do understand that People who even own a 'cross bike make up al minority of bike racers in a sparsely populated region and the organizers have to do what they have to do to try and get enough participants to make the race series worth doing.

I hope at least one of you is joking. A cross bike is the fastest whip for the job, whether it has drop bars or flat bars, disc or rim brakes, 33mm or 38mm tires. For cross, you want a light, stiff bike that climbs, accelerates, and portages well. Which isn't to say that a light hardtail mtb can't do well in a cross race, but a real cross bike is tailor-made for this specific form of racing.

Weird. I was thinking of just the opposite: I am considering using my CX bike for a local MTB race. It's pretty flat and bone dry, so I can't see that I'd be at too much disadvantage.

for the dry I would agree a CX would do well, but for mud or snow? nope.

That's what I wasn't sure about, obviously you want a lighter bike on your shoulder when you have to run with it and for the straight fast sections I do feel I gain something by going down into my drops, but in hairpin corners I'd prefer the 26" wheels and larger contact area of wider tires.

It's not about the shape of the bars, it's the weight, stiffness, and portability of the frame, the size of the wheels, the weight of the tires, and the handling characteristics.

Here's a thought experiment: Build the ideal bike for racing CX based on an MTB. It'd be a 29er hardtail, so the tires would roll faster. You don't need any suspension, which just robs your energy on climbing and accelerations anyway, and weighs down the bike. So, rigid fork. For the frame, you put enough room under the main triangle so you can shoulder it when necessary; this is important for courses with unrideable sand or mud. You trim weight on the tires to give you an optimal trade-off of weight to traction. Based on these smaller tires, you can alter the bike's geometry so it handles more nimbly.

Oh look, a cross bike.

Sure, there might be sections of a course where you'd prefer an mtb. There are minor tweaks outside of UCI rules, such as 35-38mm tires, that might make sense on some days or some courses. You personally might feel more comfortable with flat bars. But on balance, and by far, a cross bike is what you want for going fast.

Most non-championship 'cross races most places in the country allow mountain bikes, w/o bar ends. Most people in most of these races are riding 'cross bikes - for a reason. I have done a few races - deep snow, e.g., - where mountain bikes probably had the edge. If it's a good 'cross course, 9 times out of 10, the advantage will go to 'cross bikes.

And even if that wasn't true, lighten up and go race.

This is what I was looking for. This being the only 'cross series I've ever raced, I assumed that the 35mm max tire width and drop bar requirement was the norm, so I was worried all hell might break loose... or something. Since apparently the rest of the 'cross racing world gets along fine without the aforementioned regulations I guess I didn't have anything to worry about.

Over here on the wet side of the mountains, the MFG series has been allowing mountain bikes without fwd-facing bar-ends for all the years of its operation. I don't recall seeing a MTB rider on the podium all last season.
MFG isn't UCI regulated, so there's no 34mm tire max either. I recall a couple of jungle cross kinda courses where riders were rockin' 37 - 38mm tires and it may have helped on the comfort level, but not with their overall placement.