I can't say I'm a fan of Rainbow Dash being co-opted as a symbol for LGBT. Lauren Faust herself took issue with Dash being stereotyped as a lesbian by the show's detractors, since the implication is that if a girl is a tomboy who likes rainbows she's automatically gay.

Rainbow Dash (and, to a degree, the Double Rainbow video sensation) have made rainbows cool for everyone again, not just the LGBT group. When people do stuff like this though, it's simply projecting and reinforcing a stereotype on a character who was not supposed to be a stereotype.

Even though this fanbase is one of the most accommodating and diverse I've seen, stuff like this causes division because it's reinforcing a negative stereotype (that your interests and personality are directly tied to your gender and / or sexual preference).