That's not really true at all Jihnsius. Some characters Dair works as an approach and others it really doesn't. For example, Dair hits through Olimars Usmash as well as his Uair which makes it a safe approach against him. However, Snake's Utilt completely ***** it which makes it very hard to approach with it. Most characters do not have fast enough moves with the range to get past Dair.
You'd also be very surprised to know how few people know how to tech Lucas's Dair. The timing varies on the point in the air from which you used Dair which when you're approaching with it varies pretty often. Not to mention the simple fact that most players just don't come across Lucas very often. There are a lot of people who still haven't figured out that Dair has 4 hits, not 3. Of course this only gets you as far as playing people who don't know the matchup well but you'd be suprised just how far that can take you.
You also should consider that just because they can SDI or tech out of it doesn't mean it's useless. It still does 5% per hit which makes it good for racking up damage and if they do tech it, there is no way that they can punish you in time. It just means you have to find another way to get the kill move off. It is generally a low risk and possibly high reward combo. The thing to worry more about is people simply shielding and punishing Dairs when they are characters which have fast moves that hit behind them. However, this is also pretty rare.
Dair has a 6 frame disadvantage on shields. Conveniently it takes 6 frames for the opponent to drop their shield. So if you space to hit behind them, they have much fewer options and often you can shield counterattacks or use a SH Nair to eat away at their shield before they can get an attack out. Exceptions include lots of moves that come directly out of shield such as Bowser's Up B, Marth's Up B, MK's Up B etc. along with some SH aerials if the opponent can react quickly enough.
It is far from a perfect approach or perfect combo and it can be really frustrating when a character or opponent has a solid way of escaping it but overall it has a lot of positive things about it. And trust me, I was the first one to point out the fact that it can and will be teched by some opponents rendering it much weaker. But from tournament experience against a lot of competent players, I've found that it is often quite effective.