Hi,
I am no expert, and indeed there are safety issues as stated above, but I think that you will need a battery to protect from load dumps and spikes - from what im told, the battery basically acts as a 'buffer' to smooth out the supply of power to the 12v system. I would imagine the more delicate items, such as LED lighting (as they are effectively small components - diodes) will be much more susceptable to unstable supply.
Indeed, I think domestic household 12v halogen LED replacements require a specific driver as the existing 12v transformer wont be stable enough and the replacements dont last long without. I would have thought the controller for the water heater, and alarm etc fits into this category.
In computing terms (as I have some knowledge here

) an unstable supply voltage can cause all sorts of random spurious errors. As an example, we have had motherboards, memory, and CPU's replaced in a server that exibits random crashes/lockups. We eventually narrowed it down to a suspect power supply unit. We used a tool to graph what the motherboard was actually getting in voltage and the highs and lows were quite surprising. Also the same reason why its worth investing in a good quality power supply for your PC!
Cheers
Rob