Your complaints about the cross-realm instances appear to come from a lack of understanding of both the problem and the solution, so I'll try to get into both.

The problem:

Each realm has a number of instance servers. For example, Kalimdor is an instance. EK is an instance. Outland and Northrend are, too.

Now, each realm has one instance server specifically for dungeons. People are running a ton of heroics, which are run on the same instance server as 5-mans and raids. If these instance server is overstressed from trying to run too many heroics, then everyone will lag -- raiders and those in leveling dungeons, too.

So, the number of possible instances that can be created is throttled to prevent negative impact on server performance for everyone in a dungeon. But why can't the instance server handle them in the first place?

One simple reason is that a lot of the instance servers are older hardware. Another is that there are so many people playing right now due to increased interest in heroics and raiding that many servers, even recent ones, are having trouble handling them.

The solution:

Twofold. First, upgrade the hardware to realm-specific instance dungeon servers. This is in progress for most battlegroups.

Then, set up a brand new high-capacity instance server for cross-realm instance play. What this does is take the stress off of "terrestrial" instance servers, i.e. the ones that are connected to your realm alone, and instead create instances in the same way that Battlegrounds are created. It's worked very well for Battlegrounds, and there's no reason to think that it won't for instances.

Now, the idea behind this change wasn't to prevent you from pulling players straight from your realm. In fact, you can still do it, and easily. Looking at other realms for players is your choice. But if you're having trouble finding a tank, or a healer, or even (unlikely) a DPS, you can pull them from another realm in your Battlegroup and not wait half an hour spamming Trade chat to find someone.

It's an elegant solution to the problem. If you choose not to take advantage of it, you actually still benefit from it -- because most instances being created by people from your realm won't use your realm's hardware at all.