Re: spare spool vs. whole new reel
To touch up on an older thread with my experience:
I have a lower end/mid priced reel ($100 for the reel, $40 for a spool). I would, and plan on having multiple spools. I fish for steelhead and bass on the same rod and don't need to change spools on the river, or switch rods. Economically this works just fine for me, because there is no incentive to buy a new reel except for extra parts, but the reel I have is backed by a good warranty. Lord knows I'd be SOL if I kill the drag, but that's why I buy lower end reels from good manufactorers. If you encounter fishing situations where you need a couple different sinking lines and a floating line, there are two legit options depending on budget; buy 3 different rods and reels and don't waste time rigging, or buy lots of sink tips.
Personally, I'd rather have one rod and one reel that can fish for whatever fish I am after. I'm getting a 10' 8wt for steelhead and bass, and then I have a 5 weight and 3 weight for trout. I do more steelhead and bass than anything so I put most of my money there.
If you are looking for multiple spools and low prices, grey's makes, or used to make a reel that had what they called cassettes that were made of plastic that would fit over the spool, and then onto the reel frame. ~$100 would get you 4 cassettes for whatever you needed. No comment on on the product itself because I haven't had experience with it, but I'd be willing to try it if I didn't already have the reels I need.