This move would have been a lot more useful if it had been implemented one or two patches ago, but it's still a good move that should prove helpful in the upcoming season 4 by giving extra Arena points a place to go. Once a season has gone on for a few months, many avid and skill Arena players find themselves with all the Arena gear they need, perhaps even a backup weapon or two, and a stash of Arena points with nothing to spend them on. Hybrids can buy a secondary arena set for fun or in case they decide to respec, sure, but a lot of players are stuck with a lot of points and nothing to spend them on.
By the time 2.4.3 goes live, that shouldn't be a current problem, as PvPers will still be working on the points and ranking to get the new Season 4 gear. But give it a month or two, and you may find many arena players stalled and unable to get the rating for the next piece of armor. You may also find players who have reached the top yet again and purchased all the armor and weapons they feel they need. At that point, these players can now look forward to having something to spend thier arena points on in the form of the gems. The stats on the gems are even identical to the epic PvE Gems, so there's no need to feel you're spending your points to get an inferior product, either.
There's still some downsides. For one, There are no stamina or resilience gems for sale. Of course, once you have all the PvP armor, you may decide that your gem slots are better used for some more spellpower or attack power anyway. if you decide you want to maximize the healing power on your gear though, you're still going to have to shell out some gold or badges for a Crimson Spinel, which may be considered another downside as well.
Overall, though, this is a welcome change that should mean that fewer Arena Points are going to waste, and that Arena PvPers can do more of what they love without grinding gold or badges for gems.