"During my early years in Sri Lanka I did very little intensive meditation. This was not my ordination teacher’s mode of practice; he integrated regular periods of meditation into his day-to-day life. When I later practiced intensive retreats on my own, I used anapana-sati [mindfulness of breathing] as my sole meditation subject. But after some time, I found my mind became dry and rigid, and I felt the need to soften and enrich it with other types of meditation. Thus, at different times and under different circumstances, I learned the practices that constitute the “four protective meditations”: recollection of the Buddha, the meditation on loving kindness, the contemplation of the repugnant nature of the body, and the recollection of death. Throughout my life as a monk I have made extensive use of these four meditation subjects. I have also done occasional extended retreats at hermitages in Sri Lanka and elsewhere. Regretfully, though, because of my poor merits and the debilitating headache condition, I have not reached any attainments worthy of a true practitioner."
(I got this from Theravada Dhamma Blog, which I was alerted to by Bodhi Tree Swaying Blog. Here is the Wikipedia entry on Bhikkhu Bodhi.)
In my order it's always said you should alternate Mindfulness of Breathing with Metta Bhavana, and I find it interesting that only doing MOB he said, " my mind became dry and rigid, and I felt the need to soften and enrich it..." Perhaps it is a simple point, but concentration leads to emotional positivity and then spiritual death, a la Sangharakshita's System of Meditation. It's possible to develop concentration and not use it for the good.
I don't follow Bhikkhu Bodhi's ideas of literal merit. It's possible it's an quirky accident that was a cultural belief of the times. Maybe not, and maybe Bhikkhu Bodhi has a more sophisticated and deep view than I am aware of. But it doesn't seem that way in this post. Remember sometimes it's other's merit that causes things, or physical causes or systems that don't have to do with one's purity of heart. I think the point of over interpreting merit is that purity of intentions is so important, and it's hard to tie up all the disparate strands in oneself, it's not a simple effort, we are never a finished project, IMHO