I'm not against it, but it surely is not needed.
By this line of reasoning, PD wasted the time giving us the options of cars' different paint jobs. We should all be forced into one specific colour for each car.
One of GT's big draws when it first came out was an increased sense of vehicle ownership in the game. You had a garage, you "owned" these cars, you didn't just pick from a stable to complete races. Each iteration since has only built on top of this, with mileage, increased customization, and the like. A livery editor is pretty much par for the course in the genre these days, and the quite literally endless opportunities it can afford the userbase can't be ignored.
Fancy a replica? If PD were wise enough to also include a storefront-style feature, where users could sell their designs (or give them away for free, if they so choose), it'd be as easy as finding what you're looking for, and downloading it. Like I did here:
Maybe you miss some of the old GT fantasy LM cars from the PS1 days? Well, what do you know...
Getting comfortable with the editor, you can even do your own:
...or maybe get creative with your own fictitious livery:
Ideally, I'd like to see something semi-similar to Forza's livery editor, because it works nicely enough, and requires knowledge of the system to create more complicated designs. While I'd love the ability to have downloadable templates for Photoshop manipulation, I imagine that'd be a headache for PD, and could more easily lead to very offensive images.
Having it work similar to LittleBigPlanet, where we could drag and drop stickers (and hopefully, they'd wrap around different "sides" of the car) would make sense, keeping a sense of uniformity throughout Sony's marquee games. Play/Create/Share, indeed.
Besides, every time this topic is brought up, I point out the very easy solution to keep both sides happy: a simple check box in the options menu to "Disable Custom Liveries". There, you can happily race online in a pack of identi-cars, and the other users will be none the wiser. Win win!
I'd like to see templates for RM, LM, TC and PD formula car liveries. I don;t want to race against cars with penises on them.
It's still surprising how many times this line of thought pops up. I can tell you how many times I've run into that online in FM4: zero. Offensive liveries tend not to last long for two reasons:
1) Was it put on the Storefront? Chances are it'll be unpopular, so won't stay near the top of downloads (sorting by Most Downloaded almost always guarantees you looking at the best liveries), and being available on the Storefront makes it much easier for people to file a complaint about offensive material. It will be removed. That same sort of system is already in LittleBigPlanet and ModNationRacers (and I assume, LBP Karting).
2) Raced online against a swastika'd penis? File a complaint about offensive content, and having their livery removed will become the least of that person's worries.
Are there terribad, half-arsed liveries in the Storefront? Absolutely! But there's already terrible paint combinations in GT5, and as mentioned above, it's pretty easy to sort out the bad from good. Plus, a livery editor can be used for things as simple as a few small sponsorship logos trailing the front wheel, or just a license plate. Like I said; endless opportunities.