Buying an upgrade at the port is generally much cheaper than doing it in advance. Of course, it may not be available, and if the upgrade is to a suite you have lost the "in advance" services.
The concierge service is lovely. The bottom line is that you are essentially not told that something is full, even if it is your fault that you didn't plan ahead. That goes for things like Tea with Alice, Gingerbread house making (Christmas cruises only), etc. The concierge always has some 'held back" tickets. Some of the things that the concierge is being credited for (free cold meds, ibuprophen, etc) are available to anyone at Guest Services. Real dining room meals in your room, hot breakfast, ordering last day breakfast, etc. are wonderful. Ducky Williams does a concierge litho each year, and you'll get that as well as any that might apply to your cruise (he does one for the special cruises also).
OK, if money were no object, I'd permanently live in a concierge suite on a ship....but that's not reality, so the question is are the perks worth $3000. The answer to that will vary with each person.