Let me offer a different perspective (or perhaps the same perspective worded differently):
I disagree that candles are merely an aide for better concentration. Yes, they are an aid, but that is not the end of the story. I, for one, would never take a candle magic spell and eliminate the candle, much like i wouldn't take a spell that incorporates magical herbalism and eliminate the herbs. I would choose an entirely different spell based upon the ingredients, or lack thereof, that i have on-hand.
Reason being is that magic works, at least in great part, by the laws of ressonance: like equals like, and candles have some serious ressonance mojo (Google "sympathetic magic" if you have no idea what i'm talking about).
For instance, let's take a love spell and a common method of preparing a candle to be burned for that purpose:
You'll use a red candle, most likely. The color red has a strong ressonance towards love and passion, thus increasing the mojo.
You'll carve symbols such as your lover's initials, hearts, and any other sigil - again, strong ressonance toward your goal = mojo.
You'll anoint the candle with magical herb-infused oil and roll the candle in a powdered blend of love-drawing herbs. This is a blessed union between herbalism and candle magic and provides strong, strong ressonance toward manifesting your goal.
The flame of the candle burns like your passion for your lover = strong ressonance. As it burns, the color red, your herbs, oils, and symbols get released into the Universe directed towards your goal = powerfully awesome mojo.
And this is where i full-Heartedly agree with others, as the candle burns, it provides a wonderful focus for your chants, prayers, and petitions.
And this is a very simple candle magic spell. There are other candle magic spells that include figure candles to further ressonate with your goal. There is a whole body of "moving candle spells" where you actually inact your goal with candles: drawing people closer, moving people away, flusing them down a toilet, etc. In these examples, to simply eliminate the candlework is eliminating a huge percent of your mojo.
Is it possible to effect change in the world through words and willpower alone? Of course it is. Look at the huge body of work on the Law of Attraction, vision boards, positive affirmations, etc. But this is NOT candle magic, a deriviative of candle magic, nor is it sympathetic magic.
Most beginners, and experienced pracitioners alike, need all the help we can get to bring our will into fruition. We use "helpers" that have magical powers *inherent within them*: herbs, stones, colors, symbols, Diety, etc. To call these things mere tools discredits their natural powers. To join with them increases your mojo exponentially.
In short, in my opinion - take it as you will - i would not substitute out candles in a spell that really depends on candles for their efficacy. There are plenty of other non-candle spells to choose from that rely on the ressonance of your own mind, energy, and Diety; work with those.