Juno spacecraft question

Since Juno has three very large solar arrays on it to give sufficient
electrical power via sunlight (rather than RTGs) at Jupiter's distance
from the Sun, why wasn't it equipped with an ion engine to cut its trip
time down some? The arrays would generate lots of power while it's in
the inner solar system, as it will be for a lot of time till the Earth
encounter in October of 2013 slingshots it out to Jupiter:
http://www.youtube.com/watch?v=sYp5p2oL51g
Also, what's the purpose of the fabric cover over the transceiver dish?
They expecting to encounter a lot of micrometeors on the way?

Pat