Thats exactly the point why people are reluctant to donate. The budget for running Wikipedia the website is rapidly approaching single digit percentages while the foundation is hiring at startup rate.
People also want more frequent and better database dumps, scaling to account for more traffic and improve reliability, worldwide mirroring of the database servers to reduce latency across the world, a replacement of the increasingly crufty template language, etc., etc. All reasonable requests, but they require a staff of developers and sysadmins to implement.
So the angle of advertisement may be improvable, but I still think that it is worthwile to fund all their other projects through donations.
The evidence appears to be against you on this claim.