Basically, the presence of pickup
covers gives your guitar more of the mellow sound characteristic of a Les
Paul, whereas without them you can achieve a brighter, more "trashy" sound.
Of course, the result depends entirely on the pickups themselves, but you
can use this mod to accentuate the differences between, say, the bridge
and neck position pickups. Recall how one of Jimmy Page's Les Pauls
has a covered neck pickup with an open-faced pickup in the bridge position.
That's the effect he was going for.
If you want to experiment with
new sounds, find a piece of magnetic sheet metal. You can cut it
to the dimensions of your pickups and, when you lay them on top, they will
be held fast by the magnetism below. Pretty neat, huh? Actually,
when I tried it, I made it such that I could attach it to my DiMarzio FRED
using the pole pieces (I drilled 12 holes, then screwed the plate on with
the pole pieces).
The nice thing about this modification
is that it is completely and immediately reversible. You can also
switch it between the other guitars in your collection. I have only
experimented with this technique once, so you may have better or worse
luck depending on your gear and other factors. Note that there are
a lot of variables to play with if one is so inclined, including the thickness
of the metal, its iron content, etc.
You could also use a conventional pickup
cover and modify it. Watch this...