I think I must be getting soft. I commuted in South Manchester every summer when I was a student, on a steel road bike that was pretty decent for the time (531, handbuilt wheels on Mavic module E rims that I nicked off my dad's bike ...) and yes, I was faster then that I am now on my slick wheeled mountain bike, but I was significantly younger, on flatter terrain, and carrying just a packed lunch in a rucksack rather than whatever the hell is in my panniers now that makes them weigh a ton. It was all well and good, and back then I enjoyed the feeling of skittling over the rough roads (Withington was especially bad) and listening to the chain rattling against the chain stays. If you hit the rough stuff just right you sort of floated over, but that only worked for rough tarmac. Potholes were a (w)hole different matter. I think what I'm saying is that it wasn't that comfortable, and smacking into a pothole on 700x23 was unpleasant and potentially expensive. I didn't mind it being uncomfortable when I was 21 and cycling to a job I didn't *have* to do, for money I was going to spend on beer and holidays, for 3 months a year, in the summer, with no luggage to speak of. I think when you're doing it every day, whether you like it or not, whether it's sunny or not, whether you're at an age where everything seems possible or not, and on current road surfaces, I'd want a bike that is more comfortable.
A mate had the same summer job as me and our commutes merged around Mauldeth Road. He commuted on a slick wheeled rigid MTB with flat bars and bar ends, When we met on the way in occasionally, I didn't notice riding any slower, and I didn't hang about (best average was 19.9mph which I was quite pleased with given the number of traffic lights and junctions and given that my Cateye didn't do rolling average, that was journey average - that .1mph extra still bugs me a bit though!) If we left at the same time and I was doing Silly Commuter Racing (SCR) I could leave him behind a bit on the very slight downhill on Wilmslow Road - I generally rode home as hard as I could because I knew I could have a shower when I got home - but I suspect he was just spinning out because he had off-road gearing. And he generally would catch up at a set of light somewhere anyway.
And in all that, his bike was more comfortable because he wasn't running thin tyres at 120psi, safer, because he wasn't in the drops and so could see better, plus he had bigger tyre contact patches , and more practical because had we been doing it every day all year rather than for 3 months of summer, he could more easily have fitted proper mudguards and a rack. It might at that time (mid 90s) have been more steal-able because it was a very nice bike at at the time MTBs were really fashionable in a way that my steel road bike was not. That said mine eventually got nicked when I left the garage open while I nipped out to get milk in my car because it was absolutely pelting down with rain in a way that Noah would remember and I reasoned that in the 10 minutes I was out, no-one was going to come around the back of the flats. I was wrong, and the bike went. I was getting milk because that very same mate was coming around and I'd run out. He still has his MTB as far as I know. Anyway, I digress.
I think what I'm trying to say is that each kind of bike has it's own use. You can commute on a racing bike, but it's designed for racing, so it's compromised as a commuting bike. It's more fragile than most types of bike, and less comfortable. And it's harder to attach things you may want to carry to it, as well as more difficult to attach mudguards to. However, if you use a racing bike for you leisure riding, then you can of course use it commute on, it's just not ideal. But if you have that kind of bike anyway, then use it.
If, however, you're buying a bike specifically to commute on, then buy a bike more suitable for that purpose. Look for something with rack mounts, room for mudguards and slightly chunkier tyres - it doesn't have to be a flat bar bike - the Geneis Croix de Fer is often mentioned as a good commuting bike as it has mounts for all sorts, is tough as boots and yet is also versatile.
But really my top recommendation is - genuinely - get an old MTB and chuck a rack, some slicks and some mudgaurds on it. It's cheap, it's comfortable, it's reliable, the parts are cheap, it's tough and it's never going to be the first bike a thief is going to go for. Stick the spare cash you've saved in an ISA or a tracker of something, and if you ever want a "better" (more expensive) bike, you can get one. And if you don't, go on holiday with the money instead
