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If you know how hard it is to advance a skill at a certain point, you can begin to tell how long (how many tries) it should take a place to advance the skill.

For instance, using what looks like it may be DAOC's model, "Scaled skill improvement based on inverted rating":
rating ∈ [ 0, 1, 2, … 1000 ]
chance = ( rating^2 ) / MAX(rating)^2 * -1 + 1

At 400 skill, I should have an 80% chance to improve skill for any action my character performs (such as a crafting combine), or for every five combines, four will result in an improvement to character's skill. By comparison, at 900 skill, 14% of the successful combines will improve the skill, or roughly three in twenty.

The game designer can take these numbers and say, "No, I believe skill improvements should come less frequently" and can tweak the equation to produce the results (and graph) that matches what is the design requires. The designer can also begin to estimate the average total number of combines that should be needed to max that skill, determine how long each combine should take, and provide a result for how long a player should spend maxmizing that skill and compare that with the goal in the design document.

What I didn't say in the blog posting was that the chance to improve (or succeed at) a skill should be generated as a table, either when the server starts or in a data file. It can be quicker to look this value up than to calculate it, especially when working with complicated formulas or those that themselves require table lookups (sin/cos/tan, etc.).

Nebulous statements like "it's harder to advance a skill when you're already an expert" need to be backed up by some rule. In a PnP game, it's more likely to be a GM's judgement call or house rule ("To save Lagolass from the poison, the master alchemist had to find the correct plant out of season, work under a time limitation, under stress as his friend lay dying, and with less than ideal laboratory conditions. I think he learned something."). MMOs don't operate with fuzzy rules, generally, and to keep the game moving reasonably well for thousands of players, things must be simplified and described concretely. If people complained that DAOC or Eve were spreadsheets turned into games, there were good reasons for it: the design had to be modelled somewhere, someway, and predictably. Failing to adequately state or model this kind of mechanic can lead to balance problems when the game is tested or goes live. It's almost always cheaper to test something in a spreadsheet than it is to code it into a server.