Brian May uses them, and they're doing his Brighton Rock delays. Think of it as an added bonus that A: it can do extra stuff as well, might come in handy, and B: even if you still only use it for delays a new one is still €100 cheaper then a D2. I'm sure that the D2 can do things that the G-Major2 can't, like the multi-tap thing, question is, do you need that or not. Always measure your purchases by need to have vs. nice to have.
Of course they can. The days that rackmounted effects were unsurpassed by pedals is long gone. Not only can pedals nowadays deliver the same quality, they are also way more reliable as rackmounts are just not designed to be taken on the road and hauled around, being subjected to all kinds of punishment and weather conditions . The fact that Edge had to stick two of every rack unit in his rack is testament to that, he didn't do that with his pedals. Rackmounts were designed to be used in studios, static, where the temperature is always the same. That's why they are shaped the way they are. Guitarists started to take them out on the road to recreate the studio effects that had been applied to their recordings out of necessity, they didn't have an alternative.