I used Elminster a grand total of twice in my 5-year-long FR campaign.
The first time, he was called by the party (by means of his Chosen sending-like ability, which they were aware of) to help with a collapsing High Elven mythal (comprised of both regular magic and shadow magic). He told the party to run, started drawing the remaining magic into himself, and then the whole complex fell down on top of him. For all intents and purposes, he was dead for the next three years (real time), until he reappeared - together with Khelben and Halaster - after the party's climactic encounter on the newly restored Netherese flying city of Eileanar (which was plummeting to the ground at the moment, since the party destroyed the artifacts that were powering it and helped Helm slay the insane Avatar of Lathander who wanted to use the city as his base for rooting out all evil on Faerun). The three Chosen told the party to teleport away and then used their powers to stabilize the city in the air (the City is still aloft and no-one knows what's going on there).
That's how I use the high-level NPCs - they may show up to clean up, but only after the PCs have done their thing and made the decisions that needed to be made. They are way too busy dealing with their own issues to help them all the time and besides, they are operating in a league of their own, doing stuff behind the scenes and subtly manipulating the world to prevent other manipulators from getting their way.