I'm not entirely sure what you mean by link your ESO account to Steam, but I'll say this:
I bought ESO through Steam, and now I'm forever locked to launching it through Steam; last I checked there's no way around it. The standalone launcher doesn't accept Steam credentials. And I regret doing it this way, because even though it doesn't happen that often there are times where the ESO servers are up and running just fine, but only Steam players are locked out due to some technical mishap. I can hop onto my second account (not linked to Steam in any way) on the same server, no problemo.
AFAIK, there's no good reason at all to buy through Steam, and I'm not aware of any reason to link accounts after the fact if you bought the standalone version of ESO. But then again I'm not clear on what you mean by "link" accounts, so take my word with a pinch of salt.