This is a good question, Ash. I'm actually rather curious to see what others say about it myself.
I've got 2 comics that are still in the orignal poly bags they came in; 'Superman' #500, and a book I just got yesterday, '303' by Garth Ennis.
I've heard people say that leaving them in the original poly will eventually hurt them, and that some have even pulled the books out, only to find that they were damaged (Superman #500). The copy of '303' that I have, is in a clear poly bag, so you can see everything inside. I'm going to pull this one out, because it's a 'retailer copy' that's got a platinum-foil cover, and a poster that's been signed by the entire creative team. All accompanied by a COA from the publisher. I have no plans to sell this, so for me, taking it out of what could be a destructive bag, and putting it into a fresh bag, for future inspection, makes more sense to me at this time.
The 'Superman' #500 that I've got is probably one of the most MASS produced books ever, so as far as I know, there really isn't any HUGE collector value in this thing from an investment perspective. So, because of the stories I've heard about damage, I think I'll be opening this baby up, and seeing what's inside.
In regards to the whole 'slabbing' issue; if you just want to keep a copy in as good a shape as possible, then yes, breaking it out of a potentially destructive poly bag, and having it 'slabbed' sounds like a good choice. You'll have a guaranteed grade, and you'll be able to store it away for future sale/trade if needed.
Now, if it's your only copy of this particular comic, it becomes an issue of 'do you want to be able to READ it', versus, 'do you want to SELL it'?
I view the CGC services twofold thus far; as a service to give a professional grade to a comic, and sealing it to protect it, as well as ensuring that a potential buyer will know EXACTLY what kind of comic he's buying.
Would I slab a comic I wanted to read? No.
Would I slab a comic I wanted to preserve for a long period of time, OR, possibly sell later on? Yes.
Now, that's just my two cents...