As a not-yet-retired guide I can't let this one go. I've had 3 trips so far this season with clients who didn't tip; yet, all of these folks let me know repeatedly that they had their best days on the water ever and they learned a lot. I took photos and they reminded me they looked forward to my emailing them said photos. I've chalked these up to this being their first time with a guide. If I find that I am in error I would not be a happy camper. One thing is obvious: these folks didn't do their homework.
Regardless how someone feels about tipping, as it has been said before, tipping is the accepted/expected practice. Don't think for a minute that a guide who is told he or she has provided the client a great experience doesn't look for that tip. The client who doesn't tip is definitely "marked." When I loan equipment or hand out leaders, flies and other accessories - everything the client may need to be able to fish as effectively as he or she can - in addition to all of the other "stuff" mentioned in other posts, there's far more invested by the guide, whether in a boat or wade fishing, than many realize.
I would offer these suggestions to anyone who is going on a guided trip. First, if this is the first experience with a guide, do as much research as you can. Determine all that is expected/required of you, the client. Second, communicate very clearly with your guide to determine what the guide expects of you (excluding the tipping issue if you plan to tip). Third, make sure you fully understand what the guide will provide. This includes whether the guide will provide transportation, equipment, accessories, etc. Don't put either of you in a position to assume anything.
If someone is planning on hiring a guide and doesn't believe tipping is appropriate I can't urge you strongly enough to communicate to the guide that there will be no tip. At this point the guide can decide whether or not to accept to move forward or decline the trip.
A guide has invested a lot of time and effort in developing the skills necessary to provide clients the best possible experience. I believe this also includes equipping the client with additional skills to fish more effectively in the future.
Compare what a guide charges to what others charge in various technical or professional fields. I believe you'll find a guide is inexpensive in comparison.