Using a click track:
PROS
Assuming drummer lock on--assures steady tempo
Steady tempos mean you can easily align MIDI sequences to the track
Steady tempos mean cutting, pasting and looping parts is accurate and possible
Digital editing becomes simple--cut and paste verses, choruses, re-arrange the song easily
Less likely to hear fellow AE's over the Internet complain that the band changed tempos mid-song or sped up parts or whatever
CONS
99% of drummers hate playing to click tracks--only seasoned pro's do
Music can loose groove and feel
Some drummers can't play to a steady beat... mystifies me to no end why not
Risk loss of energy from the band's performances
Requires a little bit more planning and setup to get the click ready
Temptation to use sequenced synths to excess
Temptation to ruthlessly edit, cut and paste can waste a lot of time as the band and AE search for "perfection"
Allows musicians to become lazy: "alright record the riff... okay let me silence out the parts and otherwise edit it to death so it sounds perfect and then we'll loop it throughout the song" and the famous "okay record the chorus and double it and I'll cut in paste it every place there is a chorus." It can be even more ludicrous with drumming.
PERSONAL OPINION
Myself I dislike clicks. It can really iron out the feel and groove of music. Most of the classic rock songs aren't tempo perfect, but nobody cares because the performances and songs are so great. Heck, most people don't *notice* the tempo changes.
I do like drummers that can stay pretty close to being consistent with their tempo--enough that the music breathes, but not so loose that they are changing the tempo every verse and chorus, or gradually speeding up songs as they progress.
I've used clicks before and felt it added a frustration level for the band that wasn't conducive to accomplishing the recording. I've programmed songs into sequencers and had the drummer listen to it and 'double' it... time consuming but drummers are more likely to want to do this than to listen to a click track.
For rock bands the cardinal rule should always be **RECORD YOUR PERCUSSION FIRST**. I've heard of people trying to somehow align the drums up to guitar tracks later on and wondered why put yourself through that torture? (FYI: "Something in the Way" by Nirvana was recorded in that fashion because they happened to get a 'magical' performance by Cobain in the control room and Dave later had a hell of a time getting the percussion in there due to tempo variations in the playing. Adding the cello later on was a bitch too!)
In summary--clicks, a useful tool when applied properly and for the benefit of the music. Not so smart to use unless you have plenty of time or the band are total perfectionists and don't mind sounding like a drum machine. I'm not knocking drum machines though--
my Roland R8 never gets drunk, never misses practice and never misses a beat unless I program it wrong. More than I can say for any drummer.