Depends on how big your SSD is - I picked up a really cheap 60GB one a few years ago and used it to replace the Superdrive in my Early 2011 15inch MacBook Pro. I went straight into configuring it as a fusion drive because it was waaaay too small to use as a separate drive without there being issues somewhere along the line. It was the best thing I could ever have done - I now have an old (in relative terms) i7 15inch that runs at speeds my friends with 1 year old high end WinBlows laptops are envious of...
I'd say that if someone put in a big SSD alongside their HDD then yeah, it would be a 'nice to have' rather than of any significant benefit. However, if it's smaller than 80GB, then I'd say, absolutely! go for it, make it a fusion drive.
But, and it's a big but, make sure you regularly back everything up - by combining two drives to make one faster, larger one, you will also be doubling up the risk of system failure - it just takes one of them to fail for your data and OS to become inaccessible.
[Edit] PS remember to enable Trim