Absolutly right dude, we have to force ourselves to play the whole piece at the same speed (and very even in case identical note timing) regardless of the hardness of the part ... But that's only when you're practicing.

In some solos like this one in particular , accelerating or deccelarating at some parts makes special contrast which expresses some special feelings when you hear it... Exactly like other dynamics you use to express your feelings like muting,vibrato,fading in/out,etc.

These kinda expressions are only good if a good musician tries to do them expressing himself... But if somebody just overused it or do it to hide his sloppiness at some hard part of the solo , it may not sound good.
Like the vibrato technique, if realy overused with every note it will be very boring and will definately su*k ... Also the rate of vibration , and the depth (wideness) of it makes it a nice one or a crappy one.