An opening repertoire is almost never static as we evolve as players. We discover new opening ideas, learn how to better play different position-types, and continually strengthen (one hopes) different aspects of our middlegame and endgame play. After we have our initial openings chosen, what kind of process should guide the further evolution of our opening repertoire?
I believe focusing on the same two basic considerations can serve us well, even as the techniques we use in building up our repertoires become more sophisticated. Below I outline some of the different factors that will influence our choices. Aesthetic considerations are based on personal preferences, which are different for each player, while the practical considerations will affect everyone. One thing to keep in mind is that both types of considerations can change over time, as we gain a deeper understanding of the game and/or our own motivations and goals change.
- Preference for sharp, tactical positions vs. solid (i.e. less risky), strategic middlegame play
- Preference for unbalanced vs. balanced positions (in terms of pawn structure, piece play, or material)
- Preference for openings with extensive "book" analysis (e.g. Ruy Lopez - Marshall, Sicilian Dragon) vs. openings with little fixed theory
- Preference for open vs. semi-open vs. closed positions
- Preference for particular positional characteristics (e.g. fianchettoed bishops)
- Ability to understand and execute the most common middlegame plans resulting from the opening
- Ease of dealing with an opponent's deviations from "book" lines
- Time available for studying and training
- The opening's interaction with a player's middlegame and endgame strengths/weaknesses
- Opponents' opening selections and strength
- Need for achieving a win vs. a draw
- Sharp, unbalanced open positions without a huge amount of theory, need to win? Budapest Defense (1. d4 Nf6 2. c4 e5)
- Sharp, open, often unbalanced positions with a huge amount of theory, need to win? Sicilian Dragon (1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 g6)
- Little fixed theory, semi-open positions, fianchettoed bishops, need to draw? Nimzo-Larsen (1. b3)
So why do it? Usually it is a combination of the practical and aesthetic. For example, I recently decided to begin studying the Dutch Defense for several reasons. First, I wanted an opening weapon versus 1. d4 where I would have more winning chances. I have played the Slav Defense since the beginning of my tournament career with excellent results, but if White deliberately chooses a drawish line, there's little Black can do to go for a win. Second, I need to work on my attacking play and tactical ability and have been improving in those areas lately; the Dutch is very exigent in that regard. Studying and playing the Dutch will therefore contribute to my evolution as a player, as well as giving me additional practical, effective options in a tournament situation.
Also, as Wahrheit pointed out in an earlier comment, if early on in our chess careers we adopted openings that work well enough against lesser competition, but are not objectively strong enough for the opposition we currently face (or expect to face in the future), then selecting new openings will essentially be forced. In this situation, however, it is always possible to keep these types of openings - he gave the example of 1. b4 - in our repertoire as a surprise weapon against suitable opponents, so the experience should not be considered as wasted.
It's worth noting that in addition to the idea of adopting entirely new openings, evolving our choices of lines within our known openings can be just as important and should be a regular part of the training process. My current opening study methods rely on the game analysis process as well as studying opening resources such as DVDs and books.