Nice idea, robin, but completely impractical unfortunately.
Whilst it is something we ought to strive for ecologically, the weight of that extra fuel would require even more fuel to get it out there in the first place, which would endanger the whole project due to increasing costs. (larger fuel tanks, which means more weight, more weight means more fuel again, and possibly a bigger engine system.)
The simple fact remains that it is currently more cost-effective to leave them out there than bring them home again.
I favour the idea of parking them in a stable orbit. That way, there is something out there to say we have had a look, and it may be possible in a few hundred years to go and collect them again on our way to somewhere else.
Current Builds
Eaglemoss: Ecto-1, BTTF Delorean [Installing Mods]
Hachette: T800 Endoskeleton
Agora Models Shelby Cobra 427 [Plate 031]
BanDai 1:5000 Imperial Star Destroyer
AMT 1991 U.S.S. Enterprise Bridge [Installing Mods & Lights]
Finished Builds
Deagostini: R2-D2 [Never getting batteries]