Mentalists are a flexible class. They've been described as the Swiss army knives of casters, and that's very true. About the only thing a mentalist can't do with some spec or another is effectively crowd control, and resurrect. 

Now, here's the breakup of the various spell lines: 

Way of the Sun: 
Baseline damage
Baseline stun 

Illusions - Way of the Sun (Light) spec: 
Single target direct damage
Charm song
Area effect direct damage 

Way of the Moon: 
Self armor factor buff
Damage shield
Self bladeturn
Self damage absorb buff

Holism - Way of the Moon (Mana) spec: 
Heath regeneration
Single target damage over time
Power regeneration buff
Area effect damage over time 

Mentalism:
Single target heal
Single target damage over time

Mind Mastery - Mentalism spec: 
Single target major heal
Confuse
Single target mesmerize
Cure mesmerize 

Now, looking at these we see that no matter what a mentalist will have baseline healing, damage, and damage over time. And while you certainly can't spec every line effectively, there is nothing wrong with having these baseline spells on your hotbars and use them to fill in any gaps. The heals especially - you've got them, so use them. 

The spec that most people seem to go with is bringing one skill up to 50, or, at least close enough to get the last real useful spell in the line. I've never really liked this spec, to me it seemed boring, having only one line of spells to work with. Well, with an Eldritch, for example, you may really be best fully speccing in void till you hit 40.5. I have two mentalists currently, one that I'm working on that's going to be mentalism and mana spec, and the other that's light and mana. Now I've played my light/mana mentalist, and with fairly even specs, I am still able to charm a blue or yellow pet easily and solo yellows, and oranges too, if I press myself. You could as easily do light and mentalism spec, as well.  

If you're going to do a split spec like this, though, I'd recommend having a spellcrafter around, or take up the craft yourself, even if just to low levels. Most spells vary in their effectiveness unless you are fully specced in that line. For example, the charm spells in the illusions line will charm creatures 70%-110% of your level (At last check) depending on your spec percent in that line. This is true for the damage spells and damage over time, heals, baseline and spec spells. So, for example, getting items with bonuses to light will raise the level of creatures you could charm.  

Okay, now here's the two spec's I have planned for my mentalists: 

Light / Mana 
40 Light, 36 Moon, 4 Mentalism 
This spec actually looks like it might be good for many casters in general.. with bonuses you can bring your two lines to 100% spec, and even though you don't get the last spell or two, you have a much greater variety. Charm, Health regeneration, power regeneration. I plan to fire off area damage over time spells and then hammer down with area damage with this mentalist.. >:) 

Mana / Mentalism 
4 Light 46 Moon 28 Mentalism 
With this spec, you become a fairly decent backup healer, about equivalent to a warden with the same a spec, minus the warden's group heals. On top of this, you get the holism's line of heath regeneration spells, which are VERY nice to put on tanks who are taking damage, or to heal someone who's poisoned, or if you just don't have much power at the moment. You also get power regeneration, and the best area damage over time spell. You also get the mentalist's cure mesmerize, which is at 28 in mentalism. And with 4 in light, you get the first charm. Now, here's a trick with the mentalist charm. there is no level limit on the spells. The only limits are the type of creatures you can charm, and the percent of your level the creature can be. With the level 4 spell, you can only charm humanoids, and they will probably only be green, maybe blue, if you boost this line as much as you can. But, you'll still have a pet to harass enemy healers and spellcasters, or to protect yourself. 

Those just being examples, I am sure there are a few other good ones around, feel free to mess around with the spec lines. As I said, mentalists are a class that has a wide variety of tools, and if find your spec just doesn't work, then you can always respec at 20 and 40. After that, you'll need to get a respec stone, which as I understand can be very difficult. But in any case, it's a game, so why not have fun and try something new, eh? 

Classes of Camelot has a great tool called the Classes of Camelot Companion that you can download and use to help figure out where you'd like to put your points, too.