The problem is that you'll be buying a new one every year, if it lasts that long. The smaller the battery capacity, the larger percentage of that capacity you'll use for a single start. The percentage of total capacity used in a discharge cycle determines the total number of cycles you'll get, and the number of cycles decreases rapidly with deep discharges, e.g. leaving the lights on.
This battery has about 15 minutes of reserve, whereas a normal battery has around 90-100. So, you're draining the smaller battery's capacity about 6x as much as a normal battery's with every start. Not to mention that if you're coasting around with the lights on and engine off, it probably won't start at all unless you bump start it.
I disagree with this. I have an Odyssey PC680 which I have used in my CRX for over six years. I have never, I repeat NEVER had a conventional battery last THIS long. And keep in mind that it starts a Type-R motor with 11:1 compression - not, say, a D15Z1. Also remember that the stated number of 'cycles' are deep discharge cycles. This is NOT the same thing as adding up shallow discharge cycles to convert to some equlivalent number of deep cycles. Otherwise, my battery would have died LONG ago. Oh, and keep in mind that a conventional battery is not going to give you nearly the number of deep cycles.
Now it IS true that reserve capacity is going to be less than a full-sized battery. But in most cases, this is not a horribly huge problem. Unless, of course, you have one hell of a system.
One more thing. With these dry-cell batteries, you will NEVER have to worry about terminal corrosion or leaking acid EVER again.