I don't have any Majors-only decks, but have been thinking about it because I sometimes do Majors-only readings. I'm not very experienced with it but I like it a lot.
I think the advantage of using a deck that only has the majors is like someone said, that it's been conceived from the start as having only 22 cards, so it's perhaps less likely for you to find the imagery 'lacking'. Having said that, I've been using the majors of the Arcus Arcanum on their own and I didn't feel they were lacking in any way. So anyone can give this a go, you don't need a new deck
(*dodging vegetables aimed at me for trying to deprive people from a new-deck excuse*).
So why do it? It's just a very different way of reading, I wouldn't say better or worse, but the focus and the feel is different. It always gets to the bottom of things, I find Majors-only readings deeper and more to the point. Great for getting the big picture. Details are perhaps more difficult to catch, but I find it gets easier once you don't depend on the minors to automatically 'fill in the gaps'. I also tend to use less cards, usually 2-3, but up to 7 works fine for me, like the Tau-cross. On the whole for me it's more challenging and more rewarding. It's like a change of resolution, e.g. I see all the reds and blues and hues therein, but less confused and diluted, as readings with the 78 can sometimes be. Purer?
Sometimes it just feels more appropriate, but I can't really explain how I judge that in detail. Some times if I'm looking for the deeper causes or major repercussions of something, or the most important differences between two choices etc. For some readings of a more spiritual nature. Sometimes I just feel that's what the person needs. Sometimes it's what I need, so as to rely less on the actual cards and more on myself and the magic of reading...
I'd say to whoever is curious, pick out your 'strongest' Majors and give it a try. I learn a lot by doing these readings, even if in the end it's not your cup of tea, you'll come out enriched by the experience.
ps. Where I come from (Europe) I believe tarot is traditionally read with only 22 cards, Marseilles-like. I think only recently have 78 card decks become more popular.