Every creator has a right to disallow modification of their work, and as long as the EULA forbids the distribution of the modified font "software" I can see two reasons why someone would be against modification:
1) support, as many people have said. This one is easy: say you won't support modified versions in the same EULA 2) questionable design choices. If your typeface is rekerned badly, or some glyph is changed by the customer and then this modified font is used to create their logo or to print their annual report, someone might say, "Lord, this is an ugly font, who designed it? - Looks like Foo by John McTypedrawer. - He must be a terrible designer". This might happen even if your EULA forbids mentioning your name if your font is modified.
I find the shape of /a at odds with the rest of the font. Every other curve in the font is smooth and elastic, but the belly of /a has a couple bends that look like they won't disappear if the tension is released.
I can say the same about /и. It matches well with /a and looks great, but looks a bit out of place next to /c, /e, /м.
/ф looks a bit boring, the middle of the stem is too uniform in thickness. /l feels more alive.
Overall I like it a lot, looking forward to that /ю