A while ago I posted an article about regemming Haste and how I recommended not doing it. I've had a lot of time since then to think about it, and I know now what the issues and problems with the post were.
First of all, it's good to get a reminder of Haste caps. Since I don't trust my ability to do math, I did the research instead. Druid Heal! has a very easy chart to read that gives the various Haste "caps" for a Resto Druid to hit the 1-second GCD. I highly recommend looking at it before continuing on. It's what I personally use for a quick reference when I need a mental refresher as I plan out my gear.
The first thing you will notice is that the Haste cap is rather high right now. Without raid buffs and proper talent specialization into the Balance tree, it is nearly impossible to reach the cap (except for Lifebloom, which has remained at the relatively low requirements of pre 3.3). At this point in time I am running with two weapons: Trauma ,for the majority of fights in 25 man, and Mag'hari Chieftain's Staff, which is both my Mana Regen staff for extended/intensive hardmodes in 25 (with no possible downtime) and my 10 man staff, to help me reach GCD cap in a non-optimal buff environment. Unless my Haste situation improves past any possibility of raider death or 10 man non-optimization of Haste buffs, I will likely not spec out of Celestial Focus.
Some important questions one must take into account before moving any gems is: Can I reach the Haste cap? And not just any cap, but a cap that is right for my situation (10 mans, 25 mans)? Will I be able to even get reasonably close? What shape will my Mana Regen and Spellpower be in after the regemming process?
If you cannot come even close to being within the range of your cap (determined by your regular raiding composition and gear availability), all you will succeed in through regemming is gimping your Spellpower and Regen with only a marginal decrease in your GCD downtime. It takes far more Haste to make a noticeable decrease in GCD time than it does Spellpower to create a noticeable boost in a HoT tick.
If you decide that you can reach the Haste cap through regemming, do it very carefully (unless, you know, you can just throw gold around, deep pockets). It is time to start thinking ahead. These are the questions you should ask yourself:
How much Haste do I need?
What pieces of new gear do I know I will be picking up soon?
These two questions do not exist apart from each other. You must look at both in conjunction with each other. Whereas gemming for Spellpower was incredibly easy because Spellpower has no soft or hard cap for a Resto Druid, Haste does have a cap for GCD and too much extra haste is wasteful.
A main issue tends to be Druids not planning their gear out well or looking at their options. With only 2/5 pieces of t10 supplying Haste and it seeming as if a lot of drops in ICC are geared towards making the choice somewhat difficult, it's important to try to plan your purchases out and, when available to you, take extra gear. Also staying on top of the T10 bugging issue (where first Rejuvs are barely proc'ing, then being erased by other Druid's Rejuvs, then not refreshing if you HoT over it on accident but still taking your mana...it's a glitchy 4 piece) is helpful when you're deciding between Lasherweave Pauldrons and Vestments of Spruce and Fir for your next Emblem purchase (and don't be afraid to wear cloth, either! Circle of Ossus from Emblems can only be improved upon by a 25-Man Professor Putricide Hardmode drop).
So, in summary -- there is no magic answer for Haste gemming past "can I reach the cap I need?" If you can, without filling up your entire set of gear with Haste gems, then it is acceptable, and even valuable to. It is not impossible to heal without being Haste capped (I've done it before and even recently, as my staff is a new acquisition and I've been lacking my Haste buffs in 10 man) but optimally you want to be hitting a manageable, raid-appropriate cap. If you aren't there yet, you'll be okay, though. Just keep working towards it! If you cannot reach it comfortably in the gear you are in, then don't do it. But if you can, and you're raiding even somewhat seriously, you should invest.