Basically it boils down to how durable and light you want the end product to be. One way or the other you're still going to use both resin and bondo, it's just a matter of whether you mix the two together and swish it around or if you add fiberglass to make it lighter and more durable. I know in a way it doesn't make sense that adding another material to the mix would make it lighter, but it does.
If, strictly as an example, your end result will be a piece that is 1/4 thick. Cardstock obviously isn't going to be making up a great deal of that thickness, even the 110lb stuff. If you do a 1/4" coating consisting of layer after layer of rondo, basically you're just going to end up with a thick chunk of dried body filler held together solely by the hardened cardstock, which will quite likely have been sanded, etched, drilled, and in any number of other ways structurally compromised. The resin itself really only serves to make the bondo more fluid without affecting the overall weight all that much per volume, nor does it make the compound more solid or durable. And because it's essentially a solid piece of body filler, it will be very rigid, very prone to cracking or snapping with little to no flexibility at all, and quite honestly when it comes to repairing such a break, you're almost better off just scrapping it and starting over, because it is going to be a huge hassle.
On the other hand, first you harden the cardstock with resin or a thin coat of rondo to help level off some of the smaller details from the inside, the details which I'm sure Spartans 420 and C185 are referring to. Then, once it's dry, work in small areas at a time (don't try to do an entire helmet or even a full side in one go). Brush a bit of resin onto the inside and press the fiberglass onto it, and continue out and around from that spot, letting whatever you lay down get tacky (meaning it's already begun to cure and harden) before turning the piece to work on a new area (thus avoiding the chance of the fiberglass slipping or bunching). Once you've got the whole thing covered evenly (to prevent warping due to too much weight in one area and not enough to support it in others) you can add a second layer following the same method of spreading some resin, then adding fiberglass. You can achieve that same 1/4" thickness with 2 layers of fiberglass, then essentially a rondo layer (whether mixed together or resin to harden the fiberglass and bondo to cover it) to smooth the inner layer (the part that's going to be closest to your body). It's important when working with fiberglass to remember to have resin between each layer, as well as to lay down a coat before the first piece of fiberglass, and over the final layer of fiberglass so that it can be hardened all the way through.
The fiberglass adds to the thickness and is hardened by the resin while not having nearly as much weight as a straight rondo mix, and the fibers of the fiberglass will bond together along the entire piece to make it much more durable, as well as making it semi-flexible. It may still crack from too hard a hit or fall, but it's not so likely to shatter. Think of the difference between dropping a ceramic mug (the rondo-only piece) and a hard plastic cup (the fiberglass piece). Yea, there's a chance for either one to get chipped or cracked, but the mug is pretty much guaranteed to shatter into several pieces, because it has no flex in it to absorb the impact, while the plastic cup may very well bounce and not get more than a scuff on it.
If you're just doing a display piece that will get put on a stand, mounted on a wall, or set on a shelf and not see much use or movement, a pure-rondo approach might work. If you want to wear it, carry it, move around in it, or do anything beyond just looking at it, adding fiberglass for structural integrity is definitely a much better option in my opinion.