I don't like limiting myself to certain cards/colors/races/themes. I always like to explore all options and give myself the most outlets for creativity. That's just always been my preference and way of doing things, in and out of gaming.
My main opponent since 2009 did like-wise, but as his perpetual noobism was more and more apparent as time went on and I really hoped he would improve somehow, I suggested he focus only on certain factions so as to become more familiar with the game, his abilities, the cards he uses, and the rules. Well, it was a start.
A big "complaint" (read: excuse) not to play I hear from others is "I can't keep up with the releases/it's too expensive to start" (says the Magic player). Well, if you focus on one race or faction, it's not expensive. I tell them to go to deckbox, look at the cards in the game, build a deck, and get only the releases needed to make that deck. Or hell, just ask me to make it from my collection.
So, learning groups and groups with a limited collection should be encouraged to focus on a race, faction, theme, or deck type until they become intimately familiar with the rules, their opponents, and different ways decks can be built.