That's certainly true.
But getting back on topic, I would not say that Destiny was "only a test platform". At least. nothing in its backstory - either within the series itself or in the materials concerning it that were published afterwards - conclusively supports that claim.
Destiny is the only MS that carries a standard shield (it might point to the fact that it was equipped with an less reliable version of beam shields
That is pure speculation - nothing about the Destiny's beam shields as they're portrayed in the series the few times they are used even remotely hints at this possibility; I would also point out that the DOM Trooper and Legend Gundam are both equipped with the same model of beam shield, and neither machine carries a physical shield. As to why the Destiny does, that's a mystery that'll probably never be explained, though I would speculate that it's there in case it runs into weapons that can penetrate beam shields - such things do exist, as has been said above.
the standard propulsion system seems to point at a MS from Freedom's (ZGMF-X10A) generation (leg and main thrusters) rather then S.F.'s generation
I would point out that all three machines - SF, IJ, and Destiny - have conventional main and leg thrusters. So that is not a good indicator to use - in fact IJ has ONLY conventional-type thrusters, making it technologically somewhat less advanced than either of the other two in spite of its astonishingly high close-combat capabilities.
On another note, I can't remember exactly when - I think it may have been during the Orb battle - but I distinctly recall the Destiny attacking multiple enemy mobile suits at one point and in one scene(in which Destiny itself is not visible) one of the enemy machines is destroyed by a thin, blue-white beam; note that the beam rifle shots are green and those of the BFG are rather wider and colored blue-and-red like those of the Callidus series beam cannons on the Abyss, Chaos, and SF (as well as their predecessor, the Scylla used in Earth Alliance mobile suits like the Aegis and Calamity). Process of elimination would thus indicate that while never otherwise shown as such, the palm beam gun does have at least medium-long range strike capability. I confess this is NOT conclusive and I'm not saying it's definitely a fact - just what the animation seems to imply.
I also take issue with describing it as having "limited versatility", because honestly it doesn't. It's not well-equipped, but it IS equipped for combat at all ranges against a variety of possible targets - its ASBS and long-range beam cannon give it the ability to destroy targets up to and including capital ship-class at very long range or at very close quarters; Its maneuverability is high compared to all but two other mobile suits ever fielded in the series. If anything, poorly-designed as it is it's more versatile than IJ, which has ungodly high close-quarters capability but very lackluster ranged firepower(limited to its beam rifle and two laughably small beam cannons), or SF which has ridiculously high ranged firepower in space
, but is no better armed for close-range combat than the Impulse was, and in any case loses more than half its total firepower when operating on Earth. Of course the bare minimum is sufficient when the writer/director's decided you get to be the good guy now(more on that below)
As for the WoL, if anything they're far more intelligently designed than SF's, as in the latter case the WoL are obstructed by the DRAGOONs and therefore cannot be used in space unless those are deployed, and cannot be used at all on Earth where the DRAGOONs are inoperable in any case.
ultimately the problem is two-fold, and it lies outside the realm of in-universe explanations or retcons:
1. The Destiny Gundam, as much as I like it, came along right around the time that Fukuda finally decided his main protagonist wasn't gonna be the main protagonist anymore and decided to outright make him the bad guy. Ergo, its performance against the characters who upstaged him in that role seems less than stellar*. This is the reason why, among other things its WoL are made to appear essentially ineffective against SF and IJ.
2. Let's face it, it's not a very well thought-out design. There's long been a rumor floating around the 'net that Destiny's design was recycled from a rejected design for the Freedom Gundam back in Seed. That's unsubstantiated as far as I know and thus about as credible as something you'd see on /m/ (which is probably where it came from in the first place), but given #1 above, it's not implausible.
Which is the reason why [shameless plug] in the GSD rewrite I'm currently working on, I completely overhaul the Destiny Gundam design, taking the same basic concept(a machine tailored to Shinn's specific combat style, incorporating next-generation technologies and blending the capabilities of the various Impulse Silhouette packs) and creating a machine that's more in keeping with the level of technology we see by the series end - incidentally, I heavily base the design on Destiny Impulse Unit 3(the blue one) as far as its general appearance and color scheme, though very few (if any) Impulse Gundam parts will be used[/shameless plug].
(*As an aside, I find amusing that although Kira upstaged Shinn as the main character, he never once
decisively defeated him - that distinction went to Athrun.)