Not a complete answer, but a few random thoughts to start off the conversation:
1) There is another molecule that is used as a fast access store, and that is phosphocreatine which can be used to very rapidly rephosphorylate ADP in muscle. In resting muscle it is present at about 5x the level of ATP.
2) Levels of ATP are also used by cells as a regulatory input - in other words the fall in ATP levels with the onset of exercise triggers a response to replenish ATP through e.g. the breakdown of glycogen. In this view it is useful to have a final stage "energy currency" which can act directly as an enzyme substrate and whose level is a sensitive indicator of current energy demand.
3) ATP is also a substrate for RNA polymerase. If ATP was present at vastly higher levels than UTP, CTP and GTP it would probably cause errors in transcription, and might also interfere with the regulatory role of GTP binding proteins, since it would act as a competitor for binding at the GTP binding site.
4) In any case, if ATP was to be maintained at a much higher concentration for rapid use, presumably as soon as it began to be used the ATP generating systems would have to start up to try to replenish the pool. In other words things would really be no different from the way they are, but would simply operate at a higher resting level of ATP.
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