@Lomandalis:
The "just put them in raiders" argument has irked me since it became available. Having access to units made specifically for a given codex (even if some of those units are homebrewed) is preferable to me to simply taking an additional faction to unlock options you ought to have had in the first place. My main list is an eldar/dark eldar allies list*, but I still don't like my inability to deliver banshees (and other assault units to a lesser extent) without taking a dark kin hq, troop unit, and spending the unlocked FA slot on a transport.
When the new eldar codex came out, I was really hoping for something to counteract the changes to disembarkation rules. Instead, I got a gun (that is rarely a shield) that I don't have the option of leaving off my serpents and which makes my opponents go :|. Faitherun's proposed rules, or something like them, make hard-to-justify units playable again and get rid of the shield-gun that annoys my opponents. I like that.
Cost and Survivability:
Serpents are ridiculously durable. You're right to say that they're one of the best transports out there. Their armor, ability to jink, and holofields make them quite durable. The serpent shield hardly comes into that durability most of the time though. After first turn (and even then, only when my opponent goes first), my serpent rarely has access to the shield rule because it's too busy shooting at things. The cost for the serpent went up in the new codex, and a part of that increase was the serpent shield. With faitherun's changes, the shield becomes much less powerful. Adding the assault to it is worth points, but at least some of those points are covered by the debuffs to the shield as a weapon. At least, that's the I see it.
How many points do you feel the serpent shield's shooting profile is worth? How many points do you feel the assault vehicle rule is worth?
Now regarding marines, I'd first like to point out that marines that want to get into melee generally have other ways to cover ground. Assault terminators can deepstrike. Assault marines can deepstrike and move faster. drop pods are also an option. I tend to think that land raiders are a bit too pricey for what you get, but only a bit. You quoted 410 pts for a land raider filled with a marine unit. An eldar elite melee unit is like to wind up near 200 points (more for harlies with upgrades, less for banshees with upgrades), and a serpent with a gun and shuriken cannon (we'll say they don't grab holofields) is just shy of 150 points. So you're looking at 350 pts to the marines' 410. A 60 point difference is no small difference, but for those 60 points, the land raider benefits from immunity versus plasma, most eldar vehicle weapons, most tau weapons, and plenty of other weapons besides. Should land raiders be cheaper? Probably. Are serpents solid transports? For anything that isn't an assault unit, sure. Are serpents paying for the advantages they have? I think so.
Nope I'm not coming across as aggressive. That's not my intention. ^_^;
I think faitherun's changes warrant playtesting as they are. Personally I think my opponents would be ecstatic to hear that my serpent no longer has access to its usual gun.
"Wait. It's an assault vehicle now? What sort of thing are you going to assault me with?"
"Well, there are banshees. So it would be like incubi coming out of a raider, but they'd be lower strength, worse armor, and wouldn't have power from pain."
"That sounds like something I can deal with. What else might come out of there? What's the deathstar you're trying to hit me with?"
"Well, it would make it possible to use a seer council without bikes again."
"So they wouldn't have the ridiculous rerollable armor save, they'd be lower toughness, and they'd no longer be able to jink? Okay. What else?"
"I could throw scorpions at you. They're like... picture an assault marine, but he trades marine toughness for a sort of low strength hammer of wrath. "
"Mhmm. Still seems reasonable."
"And I could probably buy a serpent for one unit but put harlequins in it instead. They're as strong as incubi, but only on the charge, and only a sixth of the hits they land will ignore your armor. No power from pain either. Oh, and their saves are invulnerable but much worse."
"What else?"
"Er... storm guardians I guess?"
"Haha. Okay. I'll allow it."
Wraithblades might be nasty coming out of a serpent, but if that's the only unit that's really causing a problem, I think we can safely say they don't benefit from the assault vehicle rule due to their slowness and the fact that they have trouble perceiving the world around them.
I see faitherun's changes as a debuff for the serpent and a buff for units that aren't seeing much use right now. I would have a hard time justifying a price increase for that.