It's not about realism, per se; it's more about being practical. Skyward Sword is the first Zelda game made specifically for the Wii (as TP was a port from the Gamecube, remember?), and is making absolute use of motion controls; Skyward Sword is heavily combat focused as a result and vice-versa (they depend on one another). The freedom and control we are allowed this time around are slightly restricted by such things as a Shield Meter. This helps to balance things out.
Who knows, the way everything else works in this game, for all we know, leaving the Shield Meter out might make the game TOO easy for a lot of fans (keep in mind lack of difficulty was one of TP's MAIN faults). Leaving it out could possibly screw some other things up, as well. The Shield Meter might actually be an extremely necessary key component, and something some people would actually ask for in a hypothetical situation. Just know that it's there for good reasons.
Also, don't 'ya think it's a bit early to judge the Shield Meter on this level, soldier? I mean c'mon, we hardly got to see it, to be honest, and it appears that it might become somewhat obsolete later on in the game anyway upon obtaining the Hylian Shield...:yes: