Yes, it can be a shock at first, but each job is specialized. Classes are open to all 7, but the perks you'll gain through your job will far exceed the cross classes at a later level. As a SMN, you're focus is on dmg, not mitigation (referring to protect). Your healers should all have this, you do not need it in group content.
Intelligence is the best attribute to increase your dmg. So with that, mercy stroke at a high level will essentially do no dmg for you as it's based off attack power instead of your magic attack potency. So while that mercy stroke may seem decent enough at a low level, you'll pretty much see it's dmg will not increase for you as you level. On lvl 60 enemies, you'll likely still only be doing low dmg with it (your auto attacks will likely be doing more dmg).
And as a SMN, do not worry about losing one DoT. You will have SOOOO many more xD At your level, you should already have bio, bio II, miasma, and bane. All of these are much much better DoTs than Aero. While some crosses would be nice "like invigorate on a WAR would be awesome" the loss of certain job skills will almost never be made up for by any of the cross class skills (in the case of WAR, you would lose out on both your stances...).
Also, you can exceed the char limit by just editing your post after posting it. I usually write everything out, copy it, then delete most to post. Then just do edit and paste the entire thing again.