The way I interpret it 5% alacrity means you get on extra GCD every 20 GCDs. In solo ranked, especially 4 DPS matchups, someone is usually dead within 20 GCD.
In case there is a healer, let's say that 1 extra GCD is a thundering blast, which hits for 8k tops without a double relic proc, probably lower if you are stacking a bunch of alacrity. In that case I don't think the extra 1 GCD dmg is barely making up for a heavy surge lost.
I do believe, that in a long fight, such as regs or grp ranked, alacrity works because it speeds up dot tick and reduces cooldowns. But for solo ranked, which is the majority of PvP I play, I don't see a marginal gain from dumping surge into alacrity.