So, after a while out of the game, I've returned to pokemon and started buying 4th gen games for a DS.

However, while Heart Gold and Soul Silver are relatively easy to find, I'm having trouble finding Platinum in stores where I live. Since I prefer not to shop online, and that I notice only pearl in stock, what would be the main disadvantages of getting pearl only, and not having the extras that come with platinum?

Thanks to anyone who can answer this.

Here are the major differences that Platinum has and D/P lack:

~A larger Sinnoh dex, with a bit more variety on what Pokemon you can pick. It also features ALL of the new Sinnoh evolutions instead of just Ambipom and Weavile - so it's totally conceivable to have the new players like Dusknoir, Togekiss, or even Magnezone in your in-game roster.
~The Battle Frontier, as opposed to just the Battle Tower. It has 5 Facilities, and is identical to the HGSS Frontier.
~The graphics as a whole generally look "cleaner".
~Certain trainer sprites, particularly those of important trainers, are animated. All Pokemon back sprites also have 2 frames as well in their animation.
~Slightly adjusted difficulty and team lineups for a few trainers. For example, Cyrus is fought one more time and many of the Gym Leaders/Elites have altered team lineups (Flint now has an all Fire-type team, instead of just 2 Fire-types).
~The ability to catch Dialga and Palkia after beating the Elite Four. Giratina is fought before, at level 47, and with that comes the Distortion World.
~A bit more emphasis is placed on the Team Galactic part of the storyline. Several Galactic locations have also been redesigned.
~The Resort Area now has a Villa which you can bling out. Also it's got a pond full of Magikarp of every level.
~An abundance of move tutor moves are now available. The moves available are the same as in HGSS, except HGSS has a couple more not available in Platinum.
~The legendary birds of Kanto will Roam Sinnoh after you beat the Elite Four.
~The Regis can also be caught if you have the special event Regigigas in your party.
~Rotom and Shaymin have form differences. But you already knew that. However, in Platinum, Rotom's Secret Key is event-exclusive (and can't be traded for), whereas with Shaymin you just need to trade a (legit) Shaymin into the game and get Gracidea.

Honestly, if you already have HGSS, there is no rush to get any of the DPPt games. However, Platinum is very far superior to both Diamond & Pearl, so if you're patient, I'd wait for it to roll around. Either way you aren't missing out on much by getting or not getting it.

I own all 3 titles of DPPt, and I can honestly say that you will definitely prefer Platinum over Diamond or Pearl.