Spirit is just not as fun as other stats. Partly because mana regen is a somewhat hidden mechanic and partly because we don't have talents to make more exciting use of it. We liken this to comparing having a TBC Shadow Priest in your group or a TBC Ret Paladin in your group. In one case, your screen is filled with wonderous regen and an attractive "Misery' tooltip detailing your extra +5% damage. In the other, theres no indication the Ret Pally is even there - even the tooltips weren't updated to reflect Imp JOTC and Imp Ret wow powerleveling aura. What did the TBC ret pally give to you? You might be surprised how many players never knew, or cared. Spirit regen might 'be there' and doing something like the Ret Pally was, but just not in a compelling and exciting way like the Shadow priest was. Thus its no surprise we have little appreciation for it.

When some people say they wish they had more use for spirit, they aren't necessarily asking for overpowered regen - they just want something more useful, more tangible and more fun as a reason to appreciate the stat. A common suggestion is a passive ability (like Spiritual Attunement for Paladins) that converts a small portion of total spirit into damage/crit rating.

There are dos and don'ts of spirit, the details are as follows:

We see spirit as just that stat added on to our gear, on a piece of gear we would take regardless of whether the stat was there or not. Mage armor is that buff we use on non-DPS fights that are all about survival. Mage armor and molten armor are just different tools for the job.

With a Priest with talented divine spirit, or on the fights where we might be vital enough to warrant getting innervate, spirit is a great stat to just have in the background. But it often doesn't get to last for the full duration, because either the channeling part can get knocked down in raid damage or right after you cast it, a mob targets you with some deadly aoe that you need to move out of.