Monopods fill the gap nicely; they're lighter and shorter as a rule than tripods, and are much easier to pack and travel with. The obvious downside is that they can't support a camera in a motionless manner as a tripod can. They will steady your shot in low light, or if you're shooting with a very large lens, but you can't stand your camera on it and do a 20 second timed exposure with it.
As a result, both are handy to have. I carry a tripod for those shoots where I know I'll need to do a timed exposure, and like the monopod for dawn/dusk shoots where I need to steady the camera, and not immobilize it. Even a high-quality carbon fiber tripod will weigh several pounds; add on a ball-swivel head sturdy enough to safely manage a large lends and full size pro DSLR, and you're into a bit of weight hanging off the back of your knapsack.
If you decide to have both mono and tripods, do a bit of research on the type of base plate that you'll be using. I believe in you get what you pay for, and as such tend to save and buy the strongest, lightest and best-designed kit I can afford to have a very satisfying, long-lasting result.
However, when using certain specialty ball heads and base plates (as opposed to a more mainstream offering), the more expensive kit, albeit high-quality, can be a bit of a problem. Said specialty base plates are often machined and dovetailed to fit exquisitely into the ball head. Your camera becomes locked securely into a free-moving ball head, making for great tripod work. Wonderful.
The downside is, that much of the aftermarket kit (adapter screws, strap screws, etc) that is designed for mainstream base plate designs does not fit onto the specialized stuff. So for example, my Blackrapid R-7 shoulder strap--an outstanding strap alternative to standard vertebrae-damaging neck-straps--can use an adapter screw to allow me to switch quickly from neck strap to tripod. However, this adapter screw is made for the mainstream base plate designs, and will not fit on my exquisitely designed specialty base plate. As much as I like certain high-end tripod/monopod camera-mounting systems, once you buy into these systems, you may be unable to use widely available adapters, straps, etc. So if you want to switch your camera from one ball head that's permanently mounted on your monopod, to the panning head that's permanently on your tripod, you may find that you're stuck into a square-peg-in-a-round-hole sort of dilemma.
If you decide to add a monopod to augment your tripod, do your research and make sure that the heads and base plates are interchangeable and are of a common enough design that you can use widely available aftermarket accessories. This way, if you have one base plate screwed into your camera, it can go from the tripod to the monopod without switching base plates.
Further, if you use a carrying system like the outstanding Cotton Carrier (www.cottoncarrier.com), you can buy an adapter plate from Cotton Carrier that allows you to use your camera in their carrying system, then transition right onto a tripod/monopod. So, make sure that the heads you decide to buy for your tripod and monopod are compatible, so that when you do get an adapter accessory like the one that Cotton Carrier makes, you can move seamlessly from carrier to tripod one shoot, and carrier to monopod the next day.
And Cotton Carrier is a Canadian company. Love to see Made-In-Canada (and Made-In-the-USA) companies that employ us locals with smart designs and quality products. If you want an innovative carrying system for your pro-sized camera and lens setup, look no further:
Merry Christmas all, be safe and keep taking pix.