Presumably what you are saying is that you only purchased lenses from canon's "full frame" range rather than lenses specifically designed for entry level crop sensor bodies. I'd imagine a lot people buying a 600d would buy the cheaper crop frame lenses rather than "plan ahead" and only get full frame lenses, as I'd imagine they are usually more expensive than their crop equivalents.
I've got a mix of lenses, all work on the crop sensor and full frame - not once did I think or search for a "full frame" lens - Im not sure if there that, they're EOS compatible and thats all I wanted to know.
my 50mm f1.8 works dandy on both, initially bought for the 600d.
my Tokina 11-16mm F2.8 works great on the 600d, but I have to open up the focal length to the 16mm on the 6d otherwise I get vignetting, but it works fine. Thats one wide lens regardless of which model I slap it on.
my tamrom 70-300 was bought at the same time i bought the 600d, its terrible. Not very sharp but a good introduction for telephoto, for the price. Its gathering dust now so if anyone wants it for £50, I'd be happy to sell.
I bought a 70-200 USM L 2 for the 600d and works great, but looks much better on the 6d due to the full frame and not being cropped.
A 24-105 L works great on both camera's but looks better on the 6d, only because of not having that crop factor.
I didnt think of going to full frame, only when I thought "I'll have a go at selling images professionally", did I think I needed full frame, but more importantly 2 camera's, one as a backup. Otherwise I'd have stuck with the 600d. I wouldn't have a need for 2 camera's.
If you don't plan on going pro, I see no need to go full frame at all, to be honest, unless you have cash burning a hole in your pockets in which case, I'd rather have better lenses (to replace other lenses).
There's the crop factor and some buffering, better ISO and longer bursts of shots, I see no other major differences between that and full frame so no need to waste money when its not needed to be wasted. I had GAS but I've got over that now, only getting what I <need>, for my business. I may invest in a better led macro light than the cheapo ebay one I have, purely so I can take better photo's for pleasure on the macro side, but if a decent led ring is going to cost me as much as a dedicated macro lens, I'd rather than lens.
The higher ISO is useful for my shots at Boxing, Wrestling, Dancing and music gigs. The 6d is great at focussing in darker places than the 600d, and the 600d does get noisy after 800 but even that, at music gigs etc.. just adds "atmosphere" and the "film" look to images. Its the focussing in darker rooms I like on the 6d, more than anything else.
I have shot weddings using both camera's, school sports day's with both cameras (save me swapping lenses over all the time and missing out on a special moment). I guess everyone will have their own reasons for going full frame, I certainly wouldn't just go for it, based on people saying "its better". Its too much money just to do that.