In some casinos it is more advantageous to have separate accounts, in some it's better to have them linked, and in others it's better to combine play on one account (as you mentioned). It depends on many factors involved with the player's club such as bounceback cash, comps, or maybe mailed offers. In the best scenario, one needs to have all possible information gleaned from the experience of others to make an informed decision. For example:
Kewadin St. Ignace - Sends each of us some pretty nice bounceback cash coupons. It appears that level of play is unimportant once one passes the trigger point to rate these offers. Based on that, separate accounts are the best idea.
Harrah's - They worship at the altar of Daily Average. If you visit a Harrah's property once to earn "Diamond in a Day" by churning $30,000+ through their video poker, you should NOT go back unless prepared to play close to that level again. Should you stop in to kill a half-hour with some VP at a later date, you'll cut your daily average almost in half and screw up your rating in a big way because you'll become a $15K player instead of a $30K player. For a couple, each should consider getting their own account with two cards for each. On heavier play days, use one account - and for lighter days, use the other. A pseudo-combined account situation.
A downtown LV casino that I won't mention - We have separate accounts and don't bother with a Harrah's-style smokescreen because we each get mailed offers for room/food/beverage and bounceback that are generous enough for us. In addition, if we have friends along who could use the free room, we can both use the offers and help them out. Could a combined account get one offer that eclipses the two that we get now? Perhaps, but we're happy with the current situation and see no need to take that gamble.
There is no generalized right or wrong way to set up your accounts, it depends on the situation and casino's policies.
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Back to St. Ignace - am I to understand that you'll pass on the best TBJ deal in North America because your wife can't get a waffle????