As a rogue, I've tried both pure melee and am currently running a hybrid.
Yes the defense helps. BUT you can survive without it.
(Note: I'm using a rogue for the starting stat distribution and the availability of weapons to me, considering my limited fg. NOT for the increased defense.)
As to the difference of the +15 stats. Yes they make huge difference in the build. Particularly in the realm of defensive ability. (Explained later)
One of the factors you're ignoring is gear selection.
With 3 primary stats available to you (arguably 4 if you count heal mastery), it is *much* easier to accumulate a massive number of stats on a charm or armor than an equivalent pure melee item.
An example comparison:
Pure melee (My previous armor)
Rare tempered leather armor
Level Req: 45
Defense: 17 to 52
+50 Strength
+2% Life Steal
Cost: 300 fg
Sold at: 320 fg
Hybrid (My current armor)
Rare crystallized scale armor
Level Req: 45
Defense: 16 to 66
+28 Strength
+42 Intelligence
Cost: 250 fg
50 stats vs 70 stats (and it's cheaper).
This comparison holds at higher end gear even better. Although hybrid armor tends to be slightly rarer to find.
A second point:
Now as a hybrid build you've ignored a primary function of having a high int. DEFENSE.
The ability to heal is not the sole function of statting int. It is an important part, but far from the sole purpose.
You hard stat it for the added benefit of an increased mana pool, and the availability of low level pure melee gear.
The higher your intelligence (and dexterity) the more effective the stat is at providing a substantive defense versus it's respective damage type. (Elemental and Crits)
The defensive bonus from the two is linear. With the exception that you compare it to the mobs defense.
No, it's not an extremely cheap build. However, it's effectiveness compounds on itself as your gear gets better.
Similar to how Life Steal functions even better as your damage increases.
The primary advantage to this build, however, is the well roundedness of it - you WILL take less damage overall, by nullifying a large portion of casting damage.
This is an extremely important factor when considering chains.
As a pure melee, you take more damage during your chains, and rely solely on life steal to heal you.
The moment you run across a mob with high defense or life (or both) that casts, the damage adds up quickly ending your chain.
A heal charm is much more effective source of healing provided you have the stats and gear to back it up.
Additionally, with a Pure Melee build or Life Steal build you're pidgeonholed into relying on too few multiplying factors.
As an Intelligence based melee build you have more stats available to you AND more multipliers.
Also, it's arguable that your heal charm is just as valuable as your weapon in this build. If you slack on it, you will feel the effects.
Oh, read Wigwam's post on this subject. Try to understand the philosophy behind the build moreso than following a guide.
If I've missed anything, it's late. Point it out and I'll revisit this later to add/discuss/clarify.

Edit to add a response to your post:
Quote (TheNextBestThing @ Tue, Feb 3 2009, 04:33am)
Healing on a robed meleer is pointless if you intend to maintain good melee damage. It will never be a truly viable option..(Non cata) In catas it could work since your in the back anyway..but since you seem to dislike catas..you mine as well go pure melee and get all the damage you can because your going to need it.
and LS ofc
Having sufficient damage, well rounded defense and effective heals is more powerful than having good damage.
This style of play allows you to complete longer chains and do Wells.
With ~700 fg value in gear - at 45-46 I've experienced a 100% success ratio with wells (around 30 wells so far) on a rogue.
I challenge you to do that with a pure melee rogue setup on a similar budget.
This post was edited by maori on Feb 3 2009 04:46am