The risk of doing only tables is that you will become more used to the breathing urge, contraction, and will be able to endure through the struggle phase longer and close to your physiological limit. But unless you train properly (it means a wide variety of exercises enhancing your overall performance, not only the tolerance), it is actually quite dangerous for spear fishing and I'd strongly advice against it. It is OK for pure competitive freedivers who train and dive in well controlled environment, with buddies watching them very closely at each dive and each ascent. But at spearos or freedivers who dive alone or with buddies just close to, but nor really watching each other permanently, reducing the safety margin by increasing your tolerance may quickly turn a fatal step. The tables have place in freediver's training, but it should not be relied exclusively on them. You should better join a club with a good training program, or buy a book explaining the training methodology better. Diving a lot is a good training too, but when diving alone or with "loose" buddies, you should never ever push the limits, so the progress may be slow.
EDIT: argh, I did not see Spaghetti posting almost the same in the meantime
