Yes, there is a good reason for feat prerequisites.
Two reasons, in fact:
To clarify by indicating what build a feat is intended for (a feat that boosts Lay on Hands usually requires that you have Lay on Hands)
To balance by forcing players to dedicate more resources in order to take a more powerful feat.
I suspect the reason you're surprised to encounter this is that by and large 4e feat prerequisites are for clarity: if there's a feat we want, we probably already qualify for it. (In previous editions of D&D prerequisites were much harsher and it was common to plan our characters around qualifying for certain feats.) It's surprising to find a feat we want that we can't take in 4e.
Mastery feats need to be balanced carefully:
Feats like the Mastery set can be very powerful: they double crit range from 1/20 to 1/10, and for a build that 'fishes' for critical hits that's pretty massive (other builds wouldn't be as impressed by it, but a feat's value shouldn't be based on its usefulness to a character that wouldn't take it anyway). Consider a burst/blast wizard who attacks an average of 4 targets each round: compared to a character with expanded crit who makes one attack in the same time, the wizard with expanded crit will deal maximum damage every 3 rounds instead of every 10. If he has specialized in making his criticals awesome, this is very powerful.
In order to allow such feats to exist without unbalancing the system 4e has imposed more stringent prerequisites to take them. But...
The requirements really aren't that harsh:
Prerequisite: 21st level, Dex 15, Int 21, Wis 15, wizard
At level 21 two of a character's abilities can have increased by +6, and the other four abilities have increased by +2. This means that for Wizard Implement Expertise I can start with (before racial modifiers, assuming I take a race with +Int) a 13 Int, Dex, and Wis to qualify at 21 without focusing on either of those secondary stats or having racial bonuses to them. Roughly two out of every three wizards will have focused on at least one of them, and it's important to remember that the 4e stat generating system makes it impossible for me to start with less than a 14 in at least one stat.
A player who wants the feat should have been considering it ahead of time and planned accordingly--but even a wizard who hadn't planned to get the feat is likely to qualify. (If I want it so bad anyway, I've got two +1 stat bumps at 24 and 28 to bring my 11 up to a 13 and still enjoy the feat for the last three levels of the game.)
Sure, change it if you like.
The phrase "feat tax" gets thrown around a lot. It seems to mean that there are specific feats I need to take in order to not be a burden on my party. I generally disagree with this idea because it presupposes a specific kind of character optimization which is just one style of play rather than a philosophy of the game design.
Feat slots are limited for a reason: to force choice. There are better and worse choices, though 4e has done a surprisingly good job of limiting downright awful choices. By lifting feat prereqs and granting free feats, your group would be saying these choices aren't part of the kind of game they want to play.
And if that's your party's play style, go for it: there's no wrong way to play the game provided everyone's safe and happy. But it's not the way the game was designed, and the game wasn't designed by accident (however much I joke about the adventure writers), so we're less surprised by mechanics like these if we understand the original philosophy behind them.
Yes: curses, ability damage, and ability drain causes loss of the ability to use the feat. You don't actually lose the feat, however - you just don't gain it's benefits until you meet the prerequisite again.
From the Players Handbook 3.5 page 87.
Prerequisites
A character cannot use a feat if he or she has lost the prerequisite. For example, if your characters strength drops below 13 because of a ray of enfeeblement spell, he or she can't use the Power Attack feat until the prerequisite is met again
When you lose a feat that is a prerequisite of another feat only the abilities of the prerequisite feat are lost, because you only have to have a prerequisite feat to qualify for another feat and losing the abilities doesn't change the fact that you have the prerequisite feat, you don't actually have to be able to use the prerequisite feat to qualify for the other feat...
...So if your dexterity was dropped below 13 you would lose access to Two-Weapon-Fighting
but you would not lose access to Oversized-Two-Weapon-Fighting.
Best Answer
This uses your intelligence score, not your intelligence modifier. Therefore the DC is 25 (20 Int + 5 Proficiency modifier).