Will it do the job? probably. Is this the proper method ? probably not as most toilets have the ability (Two holes back of cistern) to mount up against the wall (check the toilet manual) assuming the walls are straight. So really, its circumstantial and the plumber will typically decide what method is best used to achieve a workable outcome.

BTW toilets are notoriously known to require extremely straight floors and walls to fit, look and function correctly. Often they need to be wedged up off the floor to accomplish a flush fit up against the wall to enable correct mounting but this could result in the pan "wobbling" over time if not done correctly.

Its not uncommon to run a bead of silicon along the cistern top (one horizontal line between cistern and wall)  before the lid goes on. This adds additional strength and maintains hygiene behind cistern. Also it can also be easily cut if the toilet ever needs to be removed. 

So, if the plumber used silicone he was either experienced or lazy. That is something you will need to evaluate based on your installation environment, toilet type etc. I hope this helps.