It's a trade-off. Focusing exclusively on hMP limits your max MP, whereas ignoring hMP means you're not resting nearly as efficiently as you could be. Some try to strike a balance, others pile on the hMP gear then piecemeal swap in MP gear as they approach cap. Do what works for you.
The major thing to consider is time. The less time spent, the more useful MP+ is. The more time spent, the more useful hMP+ is.
During an xNM situation, you will likely never /heal. xNMs take 15 minutes about, and your MP supply should try to last that 15 minutes via ethers, refresh, 2 hour abilities, etc. (This is xNM to xNM, obviously.) Again, never /healing means that hMP+ is negligible compared to MP+. It seems obvious, but it needs to be stated.
Inversely, in an EXP party situation, MP+ becomes less and less useful as parties will rarely (if ever) rest to full. Usually, the mage sets an MP amount that's needed. For example, "Pull when I'm at 200 MP." Considering the great amount of time spent resting in an EXP party, hMP+1 can be more useful than even MP+50.
For example, any given party will kill 50 monsters. Let's say in between each fight, you take 2 resting phases. So, for 50 monsters that you killed, you took 100 resting tics. That's feasible, I would say (especially for lower levels.) So, A single hMP+ would have given you 100 extra mp. MP+50 would have given you 50 MP. And, the longer you spent EXP'ing, the more that hMP gives you. Let's say you killed 100 monsters. So, the hMP+1 gives you 200 MP, and the MP+50 still only gave you 50 MP. Again, this is comparing hMP+1 vs MP+50. A very small amount compared to a very large amount.
For a realistic example, Jeff spent a couple hours farming a Pilgrim's Wand for hMP+2. Jeff is rather poor and only has MP+10 from other gear. Kevin is rich, and has no need for a Pilgrim's Wand because he has two astral rings and a slew of other gear for a total of MP+65. They party in Valkurm for 2 hours killing 100 monsters. Kevin's MP+65 gave him over those two hours, just that, 65 MP. Jeff, however, had MP+10 and MP+400 from his Pilgrim's Wand's hMP. Kevin was struggling with MP, so he used the more expensive Rolanberry Pies +1 (bear with me) for a total of +125 MP. Jeff wasn't going to use food, but he didn't want to be one-upped by Kevin, so he decided to start popping cookies for a total of hMP+7. They rested to full while waiting for new another member. For the next hour, Kevin had just what he started with, +125 MP. Jeff however had his MP+10 and a huge +700 MP from his cookies and wand.
Another thing to consider is gear swapping. Any RDM worth his salt has a Convert macro. This macro puts on all MP+ gear possible. He then uses the spare MP quickly before putting on his regular casting gear. The MP+ gear gave extra potency to his Convert. Again, the Time factor is huge here. In 30 seconds, the MP+ gear is incredibly useful, but after those 30 seconds (or less) of casting after Convert, the MP+ gear is tossed aside til the next conversion. Really, any mage can use a similar strategy. That is, he will use MP+ for as long as he can til he swaps to his usual casting gear.
A *final* consideration for MP+ gear is when weakened. My friend refers to Galka Law #1, always bring enough MP+ gear to cast raise. That is, he keeps enough gear to give him MP+150. If you're weakened, your MP pool is so small (25% of the total, I believe) that MP+ is very useful.