I really don't think people should be talking in such absolutes as "if PD wanted people to do this" because frankly nobody knows anything along those lines, and the absence of the ability in GT5 doesn't mean PD think we shouldn't do it at all, in fact they're more likely planning some way of doing this already for a future game.
Anyway, the fact that this editing is so easy to do for GT5 (copy-pasting in a plain text document is not "hacking", although it is a hack-job, so...) is why it's so rife at the moment, and I'm frankly amazed that the saves work that way at all, i.e. that the game reads what's in them without even checking if it should be possible. That said, I am pleased, to be fair, that it
is possible, because a lot of good has come of it in regards to the customisation it's unlocked - PD must look upon that aspect as a good thing, i.e. what the game can already do if only the features had a front end we could work with in the game! Plus, there's clearly a demand for it.
Cheaters will always cheat, this much should be obvious. Removing the low hanging fruit will make everyone's day a bit better, but there will still be cheaters. You cannot, 100%, stop cheaters - except, you know, by physically preventing them from playing the game, which, in practical terms, means not making the game in the first place. It is sad that the online rooms are being swamped with idiots, or children who just don't know any better, but this, in a way, is also a good thing - hopefully PD are watching and realise that the thing to do is to add more controls to the room options and search filtering so that the lunatics have a place to go and everyone else can avoid them if they want to.
The ideal case going forwards is to have the save reading patched (since I doubt they'll allow it to continue, because "piracy") so that silly stuff is ignored by the game, and cars are only allowed to have parts that belong to them and not parts that should be locked out of the game entirely - things like power "multipliers" really oughtn't be taken from the save, unless it changes with tuning, which is just bizarre. They could even encrypt it, but that could also be cracked eventually. Then PD can add in all the functionality that the editing has allowed as part of a controlled structure within the game.
All of the visual modifications that have been going on need to be added in as a feature (maybe not for GT5, but there is hope at least given how simple it should be), because they just need to be in the game already. Engine swaps can probably wait until they work out proper tradeoffs and get the full range of physics repercussions sorted, and the same for other physics-based mods.
Online requires an overhaul to make it easier to use and to set proper and wide ranging restrictions and descriptions that can be searched more effectively, which was true before these hybrids took off.
TL;DR: This is an opportunity for PD to make the game better on several levels, not to start flailing a hammer around like some power-drunk embodiment of a deranged kind-of-Thor-thing, or something. Let us spec and search for rooms more specifically and open up the customisation that is clearly already possible.
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Well said and hopefully there will be more like you out there! Then in conclusion from last night, both Modded and PD supplied cars can race with each other, fairly, respectfully, cleanly and most importantly with FUN 👍. 
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