T568A is a wiring standard, and while it does not "use" any pairs, it does specify how all four pairs must be terminated. The 802.3 standards specify how those pairs are used, and over which pairs data and/or power are transmitted.
Early versions of the Ethernet standard did indeed leave pairs 1 and 4 as spares, which did protect NICs from accidental connection to the PSTN or anything else on the first pair that might cause a problem for the electronics. (Edited to add: I don't know if this was the reason for not using the first pair or a happy coincidence, but I suspect it would have at least been a factor.)
Current versions of the Ethernet standard use all four pairs, and NICs do have some protection against accidental connection to the PSTN. In general I wouldn't worry about people accidentally connecting their NIC to a phone line even with 1000Base-T. Having said that, how effective that protection is depends entirely on how the NIC was designed and manufactured. I wouldn't test it on purpose unless you don't mind frying something in the process.
The current POE+ standard is 802.3at, and it actually uses either pins 1,2,3,6 OR 4,5,7,8, not just 7 and 8.
There are standards for the physical media, wiring and termination practices, physical interconnecting equipment, network technologies, and so on. Most of these standards depend on or are dependencies for other standards. If you throw T568 out the window, that will have an impact on any standards or technologies which depend on that standard. Ignore T568A (or B) and you won't get 1000Base-T, and perhaps not even POE. If you are going to do things in a non-standard way, then you really need to make sure you understand all of the standards and how they are related. The problem is, new technologies are often developed which rely on existing standards for physical media and terminations.
We already have a topic dedicated to the "should/should not" debate, so lets try and keep this one related to the specifics of the standards.
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